Variability in Assembly of Degradation Operons for Naphthalene and its derivative, Carbaryl, Suggests Mobilization through Horizontal Gene Transfer

In the biosphere, the largest biological laboratory, increased anthropogenic activities have led microbes to evolve and adapt to the changes occurring in the environment. Compounds, specifically xenobiotics, released due to such activities persist in nature and undergo bio-magnification in the food web. Some of these compounds act as potent endocrine disrupters, mutagens or carcinogens, and therefore their removal from the environment is essential. Due to their persistence, microbial communities have evolved to metabolize them partially or completely. Diverse biochemical pathways have evolved or been assembled by exchange of genetic material (horizontal gene transfer) through various mobile genetic elements like conjugative and non-conjugative plasmids, transposons, phages and prophages, genomic islands and integrative conjugative elements. These elements provide an unlimited opportunity for genetic material to be exchanged across various genera, thus accelerating the evolution of a new xenobiotic degrading phenotype. In this article, we illustrate examples of the assembly of metabolic pathways involved in the degradation of naphthalene and its derivative, Carbaryl, which are speculated to have evolved or adapted through the above-mentioned processes.


Introduction
Anthropogenic activities like manufacturing, industrialization and combustion of fossil fuels have led to the release of a large number of compounds in the environment, which were previously unknown to the biosphere. These compounds, known as xenobiotics, are toxic to various life forms and persist in the environment for a long period of time. Monocyclic and polycyclic aromatic hydrocarbons (PAHs) form a major group of xenobiotics. Due to the highly reduced nature, resonance stabilized structure, greater hydrophobicity, and lower aqueous solubility, they are resistant to biodegradation (recalcitrant). They act as metabolic inhibitors, endocrine disrupters [1] and further reactive epoxide formed due to metabolic activation of these compounds by microsomal enzymes interacts with DNA, causing mutation(s) or chromosomal damage [2][3][4] which may lead to cytotoxicity, genotoxicity and/or carcinogenicity [5]. Repeated application and release of these compounds in the environment has exerted a selection pressure onto the microbes, resulting in the evolution of novel/unique degradative pathways. Although processes such as volatilization, chemical oxidation and photo-oxidation contribute to degradation, bioremediation is the most efficient and cost-effective method for complete removal of these compounds [6,7]. In this review, we focus on the metabolism and various genetic elements involved in the assembly of the degradative pathways for naphthalene and its derivative, Carbaryl.
Naphthalene is ubiquitous in the environment and used extensively in the manufacture of various compounds including pesticides and plastics, as well as for domestic consumption (mothballs, insect repellent). Being hydrophobic, it partitions into the membranes [8] and gets accumulated in tissues of aquatic organisms, leading to toxicity [9,10]. In humans, exposure to naphthalene occurs via skin, oral ingestion or inhalation of fumes. Acute toxicity causes hemolytic anemia [11], whereas exposure to higher doses causes cerebral oedema and chronic renal failure [12]. Naphthalene has been classified as a potential human carcinogen based on the studies performed on experimental animals [13].
Carbaryl (1-naphthyl N-methylcarbamate), a derivative of naphthalene, is a broad-spectrum carbamate family insecticide that has been manufactured and used since 1960. It acts as a competitive inhibitor of the enzyme acetylcholine esterase of the central nervous system leading to paralysis [14]. It is highly toxic to aquatic invertebrates, amphibians, bees, earthworms and humans [15][16][17][18][19][20][21]. It has also been classified as a likely human carcinogen based on vascular tumour formation in mice [22]. In nature, various bacterial species belonging to genus Pseudomonas, Rhodococcus, Mycobacterium, Nocardia, Bacillus, Vibrio, Marinobacter, Micrococcus and Sphingomonas have been reported to degrade naphthalene and Carbaryl.

Microbial Adaptation to Aromatics and Xenobiotics
Aromatics including PAHs and xenobiotics like pesticides, being toxic and persistent, act as a selection force on the microbial community to evolve degradative pathways and possibly remove sensitive microorganisms from the population [23][24][25]. Microbes adapt to these environmental challenges and acquire the ability to degrade these compounds using the following strategies: a) Quite a few bacterial isolates have the ability to completely degrade and utilize mono as well as polycyclic aromatics like benzoate, phthalate isomers and their esters, chlorobenzoate, naphthalene, biphenyls, etc. [23,[26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41]. Such organisms harbor genes encoding all enzymes responsible for the complete metabolism of these compounds as a sole source of carbon and energy. Further, these genes could be arranged as operon(s) so as to finetune their expression to achieve optimum degradation efficiency. In a few organisms, both the carbon source as well as detoxification pathways are functional. For example, in Pseudomonas putida CSV86, 1-methylnaphthalene, which is highly toxic and used as insect repellent, is metabolized by two pathways. It is transformed and detoxified by side chain hydroxylation pathway to 1-naphthoic acid, a less toxic and more water-soluble metabolite, which is released into the medium as a dead-end product [29]. Whereas in other pathway, 1-methylnaphthalene is ring-hydroxylated and further metabolized to central carbon pathway intermediates, thus acting as sole source of carbon and energy [42]. b) In biosphere, microbial communities degrade a diverse range of pollutants. Also, specific consortia are constructed in the laboratory based on their metabolic properties and compatibility of organisms to degrade the pollutants. Members of such communities/consortia achieve complete mineralization by working together, which is mutually beneficial. Initial transformation of the parent compound by one member yields metabolite(s) which may be a dead-end product(s) for its metabolic machinery, which can then be subsequently used by other member(s). To list a few examples: consortia of Pseudomonas spp. 50552 and 50581 involved in complete degradation of Carbaryl [43]; EC20 consortia of Pseudomonas, Mesorhizobium, Achromobacter, Stenotrophomonas, and Halomonas involved in BTEX (benzene, toluene, ethylbenzene and xylene) degradation [44]; consortia of five fungal and eight bacterial isolates to degrade phenanthrene, pyrene, and benzo(a)pyrene [45]; ASDC consortia of Rhodococcus sp., Bacillus sp., and Burkholderia sp. degrading chrysene [46]. Several bacteria, algae and fungi have been reported to transform methylnaphthalene into naphthoic acid [47,48] which can then be metabolized by other microbes like Stenotrophomonas maltophilia CSV89 [49].
c) Efficient degradation of certain xenobiotics can be achieved by supplementing with specific nutrient(s). For example, isophthalate, which is used in the plastic and textile industry, acts as a competitive inhibitor for the enzyme glutamate dehydrogenase (GDH), involved in the C/N metabolism. When the growth medium is supplemented with glutamate, GDH is relieved from the and harbor a gene for integrase and/along with a short duplication of the insertion element at the other end [53,54,81].
Pathogenicity islands (PAIs), a class of GEIs associated with the virulence phenotype, were first discovered in E. coli [82] and since then a wide plethora has been uncovered. Various disease associated toxins [83,84], antibiotic resistance genes [85] and even superantigens [86] are harbored on these MGEs. The similarity between the insertion sites of PAIs and phages at tRNA loci suggests that these islands are acquired through phage mediated HGT [87]. Also, certain PAIs harbor open reading frames that show high similarity to integrase of bacteriophages [88]. In comparison, conjugative transposons contain multiple integration sites and are not confined to tRNA genes. A few ICEs/GEIs are also reported to harbor genes involved in the degradation of various aromatic compounds; for example: clc element, for the metabolism of chlorobenzoate in Pseudomonas sp. strain B13 [89]; ICEXTD for m-xylene, toluene and cumene degradation in Azoarcus sp. CIB [90]; ICECSV86 for naphthalene degradation in P. putida CSV86 [91]; ICEclc JB2 for metabolism of o-halobenzoate and ohydroxybenzoate in P. aeroginosa JB2 [92]; ICEKKS1024677 for the metabolism of polychlorinated biphenyl/biphenyl in Acidovorax sp. strain KKS102 [93]; proposed ICE harboring Tn4371, for the metabolism of biphenyl and 4-chlorobiphenyl [94]; and phn island for the metabolism of phenanthrene in Delftia sp. Cs1-4 [95]. The features of ICEs involved in aromatic compound degradation are depicted in Figure 1. The clc element is the first GEI, reported and characterized in detail from Pseudomonas sp. strain B13 and involved in the degradation of 3-chlorobenzoate [89]. ICEclcB13, ICEXTD and ICECSV86 are The clc element is the first GEI, reported and characterized in detail from Pseudomonas sp. strain B13 and involved in the degradation of 3-chlorobenzoate [89]. ICEclc B13 , ICE XTD and ICE CSV86 are associated with tRNA Gly having att site at the 3' end of tRNA (72% identity among all three att and 94% among ICE XTD and ICE CSV86 , Figure 1G). In P. aeruginosa JB2, the att site was found to be absent. The phn island involved in phenanthrene degradation from Delftia sp. Cs1-4 is structurally distinct from the clc family ICEs as it is not associated with the tRNA gene and has an SXT/R391 type mobilization system [95]. ICE KKS102 4677 harbors bph operon and shows similar arrangement as observed in clc of B13 and ICE CSV86 , consisting of direct repeats (9 bp, attL and attR region in ICE KKS102 4677) with attL at the start, followed by integrase. Also, it has been found to be integrated with the chromosome and shows low conjugal transfer frequency as observed in the case of CSV86 [93,96]. Transposon Tn4371 consists of transposase (tnpA) and phage-like integrase (int) genes [94]. Tn4371 and ICE KKS102 4677 are flanked by 8 bp and 9 bp direct repeats (55.6% identity), respectively, which are not associated with/in proximity to tRNA Gly .

Assembly of Naphthalene Degradation Pathway
Naphthalene, the simplest PAH, is used as a model compound to understand aromatic degradation pathways, enzymes and genetics. The number of microbes has been reported to metabolize naphthalene (Table 1A). The degradation is initiated by hydroxylation of one of the aromatic rings to yield 1,2-dihydroxynaphthalene, which is subsequently metabolized to salicylic acid. Generated salicylic acid is metabolized either via catechol (meta or ortho ring-cleavage) or gentisic acid to yield central carbon pathway metabolites like organic acids ( Figure 2, Table 1A). Apart from these well studied routes, isolate Bacillus thermoleovorans was reported to degrade naphthalene via phthalic acid [97]. Based on biochemical, enzyme induction and regulation studies, naphthalene degradation pathway is segmented into upper pathway (naphthalene to salicylate, nah operon) and lower pathway (salicylate to central carbon pathway either via catechol, sal operon or gentisate, gen/sgp operon).
Genes 2019, 10, x FOR PEER REVIEW 5 of 17 associated with tRNA Gly having att site at the 3' end of tRNA (72% identity among all three att and 94% among ICEXTD and ICECSV86, Figure 1G). In P. aeruginosa JB2, the att site was found to be absent. The phn island involved in phenanthrene degradation from Delftia sp. Cs1-4 is structurally distinct from the clc family ICEs as it is not associated with the tRNA gene and has an SXT/R391 type mobilization system [95]. ICEKKS1024677 harbors bph operon and shows similar arrangement as observed in clc of B13 and ICECSV86, consisting of direct repeats (9 bp, attL and attR region in ICEKKS1024677) with attL at the start, followed by integrase. Also, it has been found to be integrated with the chromosome and shows low conjugal transfer frequency as observed in the case of CSV86 [93,96]. Transposon Tn4371 consists of transposase (tnpA) and phage-like integrase (int) genes [94]. Tn4371 and ICEKKS1024677 are flanked by 8 bp and 9 bp direct repeats (55.6% identity), respectively, which are not associated with/in proximity to tRNA Gly .

Assembly of Naphthalene Degradation Pathway
Naphthalene, the simplest PAH, is used as a model compound to understand aromatic degradation pathways, enzymes and genetics. The number of microbes has been reported to metabolize naphthalene (Table 1A). The degradation is initiated by hydroxylation of one of the aromatic rings to yield 1,2-dihydroxynaphthalene, which is subsequently metabolized to salicylic acid. Generated salicylic acid is metabolized either via catechol (meta or ortho ring-cleavage) or gentisic acid to yield central carbon pathway metabolites like organic acids ( Figure 2, Table 1A). Apart from these well studied routes, isolate Bacillus thermoleovorans was reported to degrade naphthalene via phthalic acid [97]. Based on biochemical, enzyme induction and regulation studies, naphthalene degradation pathway is segmented into upper pathway (naphthalene to salicylate, nah operon) and lower pathway (salicylate to central carbon pathway either via catechol, sal operon or gentisate, gen/sgp operon).  1NH, 1-naphthol 2-hydroxylase; S1H, salicylate 1-monoocxygenase; S5H, salicylate 5-hydroxylase; C12DO, catechol 1,2-dioxygenase; C23DO, catechol 2,3-dioxygenase; GDO, gentisate 1,2-dioxygenase. ortho and meta pathway indicates the mode of aromatic ring-cleavage by these enzymes.
Metabolic diversity observed in naphthalene degradation pathways ( Figure 2) is possible due to acquisition or exchange of genetic material. Genes for naphthalene degradation were found to be present on plasmid, chromosome, transposon or ICE ( Table 1). The arrangement of genes encoding enzymes involved in naphthalene degradation is depicted in Figure 3. The degradation genes are arranged as two inducible operons: upper pathway (nah) and lower pathway (sal or sgp/gen) operon [98][99][100]. These operons are induced by salicylic acid and its analogue like 2-aminobenzoate and 2-hydroxybenzyl alcohol [100]. Among naphthalene degrading strains, the function of nahR was found to be highly conserved [101].
are proposed to be part of a defective transposon which requires Tn4653 transposase for mobilization [99]. In NAH7, the transcription for nah and sal operons is in the same direction. In P. putida strain NCIB9816-4, genes were found to be present on a conjugative plasmid pDTG1 as two operons (~15 kb apart) which are transcribed in opposite direction [102]. As compared to NAH7 and pDTG1, a non-conjugative pAK5 (IncP-7 group) from P. putida strain AK5 encodes naphthalene degradation via gentisate pathway as nah and gen/sgp operon [103]. In P. putida strain PMD-1, upper pathway genes (nah operon) were found to be located on the chromosome, while salicylate degradation genes (sal operon) were present on the conjugative plasmid pMWD-1 [104]. However, in P. stutzeri AN10, the naphthalene degradation genes (nah and sal operons) were found to be located on the chromosome and hypothesized to be recruited through transposition, recombination and rearrangement events [105,106]. Besides plasmids and chromosomes, naphthalene degradation genes were hypothesized to be present on ICE in P. putida CSV86 [91]. The draft genome analysis revealed that the nah operon is present next to tRNA Gly (along with att region) and phage-like integrase [91], suggesting the involvement of a GEI, referred to as ICECSV86 (Figure 3). This ICE is located on the chromosome and the degradation genes are arranged as nah and sal operons, which were transferred by conjugation at a very low frequency [96].  Genes responsible for upper pathway, 'nah' operon (naphthalene to salicylic acid metabolism) are depicted in green; lower pathway, 'sal' operon (salicylic acid to central carbon metabolites via catechol) are depicted in yellow; while those in pink are responsible for salicylate metabolism via gentisic acid, 'sgp/gen' operon. Genes depicted in cyan arrows indicates transposase or integrase, black boxes depict IR elements, while red box indicates tRNA gene with 'att' site, light green arrow represents the regulators of the operons and the black arrow represents the putative integral membrane transport protein. Figure is not  In P. putida G7, the plasmid NAH7 is known to harbor naphthalene degradation genes which are proposed to be part of a defective transposon which requires Tn4653 transposase for mobilization [99]. In NAH7, the transcription for nah and sal operons is in the same direction. In P. putida strain NCIB9816-4, genes were found to be present on a conjugative plasmid pDTG1 as two operons (~15 kb apart) which are transcribed in opposite direction [102]. As compared to NAH7 and pDTG1, a non-conjugative pAK5 (IncP-7 group) from P. putida strain AK5 encodes naphthalene degradation via gentisate pathway as nah and gen/sgp operon [103]. In P. putida strain PMD-1, upper pathway genes (nah operon) were found to be located on the chromosome, while salicylate degradation genes (sal operon) were present on the conjugative plasmid pMWD-1 [104]. However, in P. stutzeri AN10, the naphthalene degradation genes (nah and sal operons) were found to be located on the chromosome and hypothesized to be recruited through transposition, recombination and rearrangement events [105,106]. Besides plasmids and chromosomes, naphthalene degradation genes were hypothesized to be present on ICE in P. putida CSV86 [91]. The draft genome analysis revealed that the nah operon is present next to tRNA Gly (along with att region) and phage-like integrase [91], suggesting the involvement of a GEI, referred to as ICE CSV86 (Figure 3). This ICE is located on the chromosome and the degradation genes are arranged as nah and sal operons, which were transferred by conjugation at a very low frequency [96].

Assembly of Carbaryl Degradation Pathway
Carbaryl has been used extensively in the agriculture sector since 1960. Repeated application has led microbes to adapt and utilize Carbaryl as the sole source of carbon and energy (Table 1B). The degradation pathway is initiated by hydrolyzing the ester bond by the enzyme Carbaryl hydrolase (CH) to yield 1-napthol. Generated 1-naphthol is ring-hydroxylated by the enzyme 1-naphthol hydroxylase (1NH) yielding 1,2-dihydroxynaphthalene, which is further metabolized via salicylic acid and gentisic acid. Few Carbaryl degraders also metabolize salicylic acid via catechol route [44] (Table 1B and Figure 2). The metabolic steps involved in the conversion of 1,2-dihydroxynaphthalene to salicylic acid are common for naphthalene and Carbaryl degradation ( Figure 2). Interestingly, in naphthalene degraders salicylic acid is predominantly metabolized via catechol, whereas in Carbaryl degraders, it is metabolized mainly through the gentisic acid route (Table 1B and Figure 2).
A large number of genes involved in Carbaryl degradation have been proposed to be plasmid borne [121]. Arthrobacter sp. RC100 harbors three plasmids (pRC1, pRC2, and pRC300) of which conjugative plasmids, pRC1 and pRC2, encode enzymes for Carbaryl degradation up to gentisic acid. Enzymes involved in the conversion of gentisic acid to central carbon metabolites are encoded by genes located on the chromosome [115]. A Rhizobium sp. strain AC100, which transforms Carbaryl to 1-naphthol, harbors a plasmid pAC200 carrying gene cehA encoding CH as a part of Tnceh transposon flanked by insertion element-like sequence (istA and istB, Figure 4). istA and istB, which comprise the ISRsp3 element, showed homology to IS1600 element from A. eutrophus NH9 and of IS1326 from P. aeruginosa of the IS21 family. Treating cells with plasmid curing agents like mitomycin-C resulted in deletion of cehA from the Tnceh leaving behind the insertion elements as a part of plasmid pAC200d due to homologous recombination among ISRsp3 [116]. This depicts the instability of the system and loss of catabolic genes by the activation of IS elements. Sphingomonas sp. strain CF06, which degrades carbofuran and its derivatives (including Carbaryl), harbors five plasmids-pCF01, pCF02, pCF03, pCF04, and pCF05. Plasmid curing and conjugation experiments revealed that the degradation property was associated with plasmid(s). Southern hybridization studies revealed a high degree of sequence similarity among pCF01 and pCF02, and pCF03 and pCF04, indicating gene duplication, which could be due to lack of positive regulatory system. Various IS elements were found to be present on pCF01, pCF02 and pCF03. The presence of active IS elements as well as gene duplications indicate that the system is in an early stage of evolution [67]. In Carbaryl degrading consortia of two Pseudomonas spp., the strain 50581 harbors a conjugative plasmid, pCD1 (50 kb) which encodes for CH, while chromosomally located genes from strain 50552 encode enzymes for 1-naphthol degradation [43].
Genes 2019, 10, x FOR PEER REVIEW 9 of 17 loss of catabolic genes by the activation of IS elements. Sphingomonas sp. strain CF06, which degrades carbofuran and its derivatives (including Carbaryl), harbors five plasmids-pCF01, pCF02, pCF03, pCF04, and pCF05. Plasmid curing and conjugation experiments revealed that the degradation property was associated with plasmid(s). Southern hybridization studies revealed a high degree of sequence similarity among pCF01 and pCF02, and pCF03 and pCF04, indicating gene duplication, which could be due to lack of positive regulatory system. Various IS elements were found to be present on pCF01, pCF02 and pCF03. The presence of active IS elements as well as gene duplications indicate that the system is in an early stage of evolution [67]. In Carbaryl degrading consortia of two Pseudomonas spp., the strain 50581 harbors a conjugative plasmid, pCD1 (50 kb) which encodes for CH, while chromosomally located genes from strain 50552 encode enzymes for 1-naphthol degradation [43]. Soil isolates Pseudomonas spp. strains C4, C5pp and C6 utilizes Carbaryl via salicylic acid and gentisic acid (Figure 2, [30]). Based on metabolic studies like enzyme induction, cell respiration and biotransformation, the Carbaryl degradation pathway was hypothesized to be divided into upper, middle and lower segments; and genes are probably arranged as three distinct operons induced by respective carbon source [122]. The degradation property was found to be very stable and could not be cured by nutritional conditions or chemical agents. Southern hybridization analyses indicate genes encoding degradative enzymes are located on the host genome [122]. The draft genome sequence of Pseudomonas sp. C5pp, primer walking, and gap filling PCR studies yielded a 76.3 kb sequence, referred to as Supercontig-A, harboring all genes required for Carbaryl degradation ( Figure 5), [123,124]. Genes encoding enzymes responsible for the conversion of Carbaryl to salicylic acid (upper operon), salicylic acid to gentisic acid (middle operon), and gentisic acid to central carbon metabolites (lower operon) are probably under the regulation of transcription regulators encoded by mcbG, mcbH and mcbN, respectively ( Figure 5).  Soil isolates Pseudomonas spp. strains C4, C5pp and C6 utilizes Carbaryl via salicylic acid and gentisic acid (Figure 2, [30]). Based on metabolic studies like enzyme induction, cell respiration and biotransformation, the Carbaryl degradation pathway was hypothesized to be divided into upper, middle and lower segments; and genes are probably arranged as three distinct operons induced by respective carbon source [122]. The degradation property was found to be very stable and could not be cured by nutritional conditions or chemical agents. Southern hybridization analyses indicate genes encoding degradative enzymes are located on the host genome [122]. The draft genome sequence of Pseudomonas sp. C5pp, primer walking, and gap filling PCR studies yielded a 76.3 kb sequence, referred to as Supercontig-A, harboring all genes required for Carbaryl degradation ( Figure 5), [123,124]. Genes encoding enzymes responsible for the conversion of Carbaryl to salicylic acid (upper operon), salicylic acid to gentisic acid (middle operon), and gentisic acid to central carbon metabolites (lower operon) are probably under the regulation of transcription regulators encoded by mcbG, mcbH and mcbN, respectively ( Figure 5).
The draft genome (6.15 Mb) analysis revealed the presence of 42 MGEs and 36 GEIs, out of which 17 MGEs were located in Supercontig-A (76.3 kb), indicating this region could be a hotspot for genome alterations [123,124]. The upper and middle operon region showed lower G+C content (54%) compared to the genome (62.65%) of strain C5pp. Whereas, the lower operon region showed comparable (60%) G+C content. This skewing of G+C content suggests that genes of the upper and middle pathways probably have a different ancestral origin and have been acquired through HGT events and assembled in proximity to each other. This is evident from the fact that 40% of MGEs are located in the Supercontig A (which is 1% of the total genome) [123,124]. Of the 36 GEIs, 3 were found to be a part of Supercontig-A. The upper pathway genes, except the regulator mcbG constitute a single GEI (13.8 kb), whereas the middle (6.5 kb) and the lower (9.6 kb) pathway genes, including their respective regulators, were established to be parts of two distinct GEIs ( Figure 5). Interestingly, P. putida XWY-1, isolated from waste water in Shangong, China, 4750 km away from the site of isolation of the strain C5pp (Mumbai, India), showed a similar gene arrangement and nucleotide sequence for the genes involved in Carbaryl mineralization; however, these genes were reported to be plasmid borne [120]. respective carbon source [122]. The degradation property was found to be very stable and could not be cured by nutritional conditions or chemical agents. Southern hybridization analyses indicate genes encoding degradative enzymes are located on the host genome [122]. The draft genome sequence of Pseudomonas sp. C5pp, primer walking, and gap filling PCR studies yielded a 76.3 kb sequence, referred to as Supercontig-A, harboring all genes required for Carbaryl degradation (Figure 5), [123,124]. Genes encoding enzymes responsible for the conversion of Carbaryl to salicylic acid (upper operon), salicylic acid to gentisic acid (middle operon), and gentisic acid to central carbon metabolites (lower operon) are probably under the regulation of transcription regulators encoded by mcbG, mcbH and mcbN, respectively ( Figure 5).   In Pseudomonas sp. strain C5pp, the upstream region of the upper pathway genes showed class-I integron features, which showed high identity (95-99%) with Tn6217 [124]. Other features present were transposase, 25 bp left-end repeat (92% homology to IR i ), attI site, 5 and 3 conserved segment, resistance to streptomycin and two additional genes. The regulator mcbG is flanked by truncated transposases, which showed a similarity to ISPa20 of IS3 family at the left end and ISPst7 of IS5 family at the right end. The presence of partial transposase sequences indicate that their function has probably been lost due to decay linked recombination events and these might be the leftover sequences of previously functional ICE. The region consisting of middle pathway genes, mcbIJKL, flanked by IS21 family insertion repeats, exhibits class-I composite transposon like features and is referred to as the catabolic transposon TnC5ppsal [123]. Members belonging to class-I composite transposon have been known to harbor gene clusters involved in degradation of xenobiotics [25]. The transposases present at the 3' end of the cluster show a high degree of similarity to the IS110 and IS5 family, reverse transcriptase, group-II intron D1-D4-2, and leucine-zipper class of integrase. As depicted in Figure 5, the insertion elements of the IS21 family are present in inverted orientation, which could be the reason for the stability of the catabolic transposon in the genome of strain C5pp. Also, it has been reported that the increase in size of the intervening sequence of the composite transposon lowers the transposition frequency [123,124]. The lower pathway genes encoding for catabolism of gentisic acid to central carbon metabolites, are hypothesized to be a part of class-I composite transposon, which is bordered by non-identical insertion elements showing sequence similarity with ISPa1635 and IS481 [123,124]. In addition to this, the prediction of the probable ancestral origin of the genes using the IMG-JGI server indicates that mcbBC, mcbEF, and mcbIJKL were probably derived from beta-proteobacteria; mcbA and mcbM from alpha-proteobacteria; and mcbD, mcbGH and mcbNOPQ from gamma-proteobacteria.
The steps involved in the conversion of 1,2-dihydroxynaphthalene to salicylate appears to be common in both naphthalene and Carbaryl degradation pathways. Pair-wise sequence alignment (nucleotide as well as amino acid) of genes nahC, nahD, nahE and nahF (encoding these metabolic steps) from various naphthalene degrading Pseudomonads ( Figure 3) and genes mcbB, mcbD, mcbE, and mcbF from Carbaryl degrading Pseudomonas sp. C5pp ( Figure 5) revealed higher (~90-100%) identity amongst 2-hydroxybenzylidene pyruvate hydratase-aldolase as well as salicylaldehyde dehydrogenase encoding genes while 2-hydroxychromene 2-carboxylate isomerase showed moderate (~44-52%) identity. The phylogenetic analysis of Carbaryl degradation genes has been described in detail by Trivedi et al. [123,124]. Interestingly, 1,2-Dihydroxynaphthalene dioxygenase (12DHNDO) from strain C5pp, which belongs to class-II extradiol dioxygenases displayed low (4-10%) identity with 12DHNDOs from naphthalene degraders (class-I) as compared to protocatechuate 3,4-dioxygenase from lignin degraders Pseudomonas humi (74%) and Burkholderia sp. LIG30 (73%) [123,124]. Among various naphthalene degraders, irrespective of location, these genes displayed high (86-98%) identity. In strain C5pp, genes encoding CH and 1NH were hypothesized to be acquired from other degraders. Phylogenetic analysis revealed that CH from C5pp belongs to a new family of esterase with new conserved motif [124]. Enzyme 1NH from strain C5pp, which showed 47% activity on 2,4-dichlorophenol as substrate, yielded 55% identity with 2,4-dichlorophenol 6-monoxygenase from Paraburkholderia zhejiangensis [123,124]. This suggests that the strain has probably acquired the relevant gene, followed by mutations to evolve the enzyme, which will now accept 1-naphthol as its primary substrate. All of these observations indicate that the Carbaryl degradation genes must have been acquired through HGT followed by multiple transposition events leading to integration into the genome, giving a stable degradation phenotype in Pseudomonas sp. strain C5pp even in the absence of selection pressure.
Besides HGT events and assembly of genes in the form of operons, Pseudomonas sp. C5pp shows cellular level adaptation by compartmentalization of enzymes to tackle the toxicity of 1-naphthol [125,126]. This adaptation helps in degradation of higher concentration of Carbaryl (1% tested so far). The ability to metabolize Carbaryl at high concentration can be attributed to the presence of a low affinity CH (K m , 100 µM) in the periplasm of the bacterium [126]. This enzyme catalyses the conversion of Carbaryl to 1-naphthol (more toxic and recalcitrant than Carbaryl) in the periplasm, minimizing the interaction of 1-naphthol with cytoplasmic components. 1-Naphthol, transported across the membrane through diffusion and partition processes into the cytoplasm, is ring-hydroxylated by high affinity cytosolic 1NH (K m , 10µM) to 1,2-dihydroxynaphthalene and subsequently metabolized to central carbon pathway intermediates. The compartmentalization of enzymes involved in the degradation suggests a successful strategy evolved by Pseudomonas sp. strain C5pp for efficient degradation of Carbaryl at high concentration.

Conclusions
Bacteria, which are present ubiquitously in nature, with a simple unicellular structure and ability to grow faster, have made their niches very dynamic, metabolically as well as genotypically plastic, and hence hot spots for the evolution of new trait(s). During the process of adaptation to strenuous conditions, microbes might acquire new genes and functions to enhance survivability. In addition, they might mutate already existing and/or acquired genes. The former approach is mediated through MGEs, which are the major players responsible for genome plasticity. Microbes evolve genes for novel pathways encoding xenobiotic degradation by the above mechanisms, which are further assembled as transcription unit(s)/operon(s), so as to finetune their regulation. Thus, only the required enzymes of the degradative pathway or segment of the pathway are activated by the respective carbon source. Further, the mutations in the acquired gene(s), shuffling/rearrangement of genes and compartmentalization of metabolic steps grant an additional advantage to the organism to adapt and utilize the compound more efficiently, as observed in the case of Carbaryl metabolism by Pseudomonas sp. C5pp. Apart from plasmids and transposons, the genes for aromatic degradation have also been found to be present as a part of GEIs/ICEs in the genome of various organisms including Pseudomonas. Integration of the degradation property into the host genome may impart additional stability to the phenotype, as generally the plasmids are lost in the absence of selection pressure. Therefore, by employing various strategies, microbes display an ability to adapt to challenging environments.
Author Contributions: All three authors contributed equally towards conceptualization, writing original draft, review and editing. P.S.P. acquired funds through various government agencies to support the research activities.
Funding: PP acknowledges research grants from BRNS, DBT and DST, Govt. of India.