Effects of mrpigG on Development and Secondary Metabolism of Monascus ruber M7

Monascus pigments (MPs) have been used as food colorants for several centuries in Asian countries and are now used throughout the world via Asian catering. The MP biosynthetic pathway has been well-illustrated, but the functions of a few genes, including mrpigG, in the MP gene cluster are still unclear. In the current study, in order to investigate the function of mrpigG in M. ruber M7, gene deletion (ΔmrpigG), complementation (ΔmrpigG::mrpigG) and overexpression (M7::PtrpC-mrpigG) mutants were successfully obtained. The morphologies and biomasses, as well as the MP and citrinin production, of these mutants were analyzed. The results revealed that the disruption, complementation and overexpression of mrpigG showed no apparent defects in morphology, biomass or citrinin production (except MP production) in ΔmrpigG compared with M. ruber M7. Although the MP profiles of ΔmrpigG and M. ruber M7 were almost the same—with both having four yellow pigments, two orange pigments (OPs) and two red pigments (RPs)—their yields were decreased in ΔmrpigG to a certain extent. Particularly, the content of rubropunctatin (an OP) and its derivative rubropunctamine (an RP) in ΔmrpigG, both of which have a five-carbon side chain, accounted for 57.7%, and 22.3% of those in M. ruber M7. On the other hand, monascorubrin (an OP) and its derivative monascorubramine (an RP), both of which have a seven-carbon side chain, were increased by 1.15 and 2.55 times, respectively, in ΔmrpigG compared with M. ruber M7. These results suggest that the MrPigG protein may preferentially catalyze the biosynthesis of MPs with a five-carbon side chain.


Introduction
Monascus spp., a type of medicinal and edible filamentous fungus, have been used to produce fermented foods and medicine in Asian countries, such as China, Japan and the Korean Peninsula, for nearly 2000 years [1,2]. At present, their fermented products, such as red fermented rice (RFR), also called red yeast rice, Anka, Hongqu, red Koji, and red mold rice, are widely used as food additives and nutraceutical supplements worldwide, owing to their production of beneficial secondary metabolites (SMs), mainly including Monascus pigments (MPs), monacolin K (MK) and γ-amino butyric acid [1,3,4], even though some strains of Monascus spp. may produce citrinin (CIT), a kind of nephrotoxic mycotoxin [5,6].
MPs are a complex mixture of compounds with a common azaphilone skeleton [7], and more than 110 MP components, most of which possess a five-or seven-carbon side chain, have been identified to date [8]. Despite their large-scale utilization and research, the biosynthesis of MPs remains inadequate to a certain extent. In the last 10 years, a unifying picture of the biosynthesis of MPs has emerged as a result of genome sequencing and the functional analysis of MPs' biosynthetic gene clusters. In 2017,

Cloning and Analysis of the mrpigG Gene
A pair of primers, pigG F1-pigG R1 (Table 2), were designed to amplify the mrpigG gene, using Oligo 6 software (http://www.oligo.net/). PCR was carried out to amplify the mrpigG gene from the genome of M. ruber M7. Amino acid sequences encoded by mrpigG were predicted using Soft Berry's FGENESH program (http://www.softberry.com), and the MrPigG functional regions were analyzed using the Pfam 33.1 program (http://pfam.xfam.org/). Homology of the deduced amino acid sequence was analyzed using the BLASTP program on the NCBI website (http://blast.ncbi.nlm.nih.gov/Blast.cgi).
The gene complementation structure possessed 5 flanking, together with mrpigG open reading frame (ORF) regions (1823 bp, amplified with the primer pair pigG c5F and pigG c5R), neomycin phosphotransferase resistance gene (neo, 1221 bp, amplified from plasmid pKN1 with the primer pair G418 F and G418 R) and 3 flanking regions (860 bp, amplified with the primer pair pigG c3F and pigG c3R), consecutively.
The gene overexpression construct carried 5 flanking regions (969 bp, amplified with the primer pair pigG ov5F and pigG ov5R), neomycin phosphotransferase resistance gene (neo, 1221 bp, amplified from plasmid pKN1 with the primer pair G418 F and G418 R), the trpC promoter (373 bp, amplified from plasmid pSKH using the primer pair PtrpC F and PtrpC R) and the mrpigG gene together with 3 flanking regions (1621 bp, amplified with the primer pair pigG ov3F and pigG ov3R), consecutively.
Three abovementioned structures were separately cloned into pMD19-T (Takara, Dalian, China), then all the cloned fusion DNA fragments and plasmid pCAMBIA3300 were digested with KpnI and HindIII, and were ligated by T4 DNA ligase to generate plasmids pCPIGG, pCCPIGG and pCOPIGG, respectively. Then three plasmids were separately transformed into Agrobacterium tumefaciens EHA105 using a freeze-thaw method. Then, three A. tumefaciens EHA105 clones containing pCPIGG, pCOPIGG were incubated for transformation with M. ruber M7 to yield the mrpigG disruption strains (∆mrpigG), mrpigG and overexpression strains (M7::PtrpC-mrpigG), respectively. Meanwhile, the A. tumefaciens EHA105 clones containing pCCPIGG were incubated for transformation with the ∆mrpigG strain to generate the mrpigG complementation strains (∆mrpigG::mrpigG).

Real-Time qPCR Analysis
The expression of mrpigG in the selected ∆mrpigG, ∆mrpigG::mrpigG, M7::PtrpC-mrpigG strain and the wild-type strain (M. ruber M7) was analyzed by real-time quantitative PCR analysis (RT-qPCR). One milliliter freshly harvested spores (10 5 cfu/mL) of each strain were inoculated into PDA medium and incubated at 28 • C, and samples were taken every other day from the 3rd day to the 11th day. RT-qPCR was performed according to the method described by Liu et al. [14]. GAPDH was used as a reference gene. The primers used in these analyses are listed in Table 2.

MP and Citrinin Analyses
Previous studies have revealed that MPs accumulate mainly in the mycelia, where citrinin exists in media [16]. Thus, intracellular MPs and the extracellular citrinin were detected. One milliliter of freshly harvested spores (10 5 cfu/mL) of each strain were inoculated on PDA plates covered with cellophane membranes, and incubated at 28 • C for 11 days. The samples were taken every 2 days from the 3rd day to the 11th day of culture to measure the MPs and CIT production [5]. Twenty milligrams of freeze-dried mycelia or media powder were suspended in 1 mL 80% (v/v) methanol solution, and subjected to 30 min ultrasonication treatment (KQ-250B, Kunshan, China).
Then, MPs were separated by an ACQUITY UPLC BEH C18 column (2.1 mm × 100 mm, 1.7 µm, Waters), and detected on a Waters ACQUITY UPLC I-class system by UV-Vis spectra (Waters, Milford, MA, USA). A gradient elution was performed with the mobile phase including solvent A (acetonitrile), solvent B (ultra-pure water) and solvent C (0.1% formic acid in water) with a flow rate of 0.3 mL/min and an injection volume of 2 µL. The gradient elution was performed as follows: firstly, Solvent A/B/C maintained 35:55:10 for 3 min, then the content was changed to 75:15:10 for 15 min, followed by adjusting to 90:0:10 for 5 min. Subsequently, the solvent was changed to 35:55:10 for 5 min. Finally, the column was equilibrated with 35:55:10 for 3 min.
The citrinin was detected on a Waters ACQUITY UPLC system by means of a fluorescence detector (Waters, Milford, MA, USA) A gradient elution was performed with the mobile phase including solvent A (0.1% formic acid in water) and solvent B (acetonitrile) with a flow rate of 0.3 mL/min and an injection volume of 2 µL. The gradient elution was performed as follows: Solvent A/B was first maintained at 90:10 for 3 min. Secondly, the content of solvent A was decreased from 90% to 30% for 7 min. Then, the amount of solvent A was decreased from 30% to 10% for 0.01 min and was maintained for 3 min. Finally, the amount of solvent A was increased from 10% to 90% for 3 min. The temperature of the chromatographic column and samples were maintained at 40 • C and 4 • C, respectively.

Sequence Analysis of the mrpigG Gene in M. ruber M7
A 1.24-kb fragment containing the putative mrpigG homolog was successfully amplified from the genomic DNA of M. ruber M7. Sequence prediction of mrpigG by SoftBerry's FGENESH program revealed that the putative mrpigG gene consists only of an 822 bp open reading frame (ORF) which consists of 1 exon and encodes 273 amino acids. A database search with the Pfam 33.1 program showed that MrPigG pertains to the serine hydrolase family (FSH). A database search with NCBI-BLAST demonstrated that the deduced 273-amino acid sequence encoded by mrpigG shares 93.01% similarity with the amino oxidase/esterase of M. pilosus (GenBank: AGN71609.1), 69.26% similarity with serine hydrolase FSH of Penicillium occitanis (GenBank: PCH03976.1), and 55.72% similarity with the serine hydrolase-domain-containing protein of Pseudomassariella vexata (GenBank: ORY68205.1).

Verification of the mrpigG Deletion, Complementation and Overexpression Strains
After the plasmid pCPIGG was transformed into M. ruber M7, transformants with hygromycin B resistance were obtained and verified by PCR, and three of 115 were transformant mrpigG-deleted strains. As shown in Figure 1c, no DNA band was amplified when genomic DNA of one mrpigG disruptant named ∆mrpigG was used as template with primers pigG F2-pigG R2, although a 0.5-kb product appeared using the genome of the wild-type strain M. ruber M7. A 2.14-kb fragment of the hph gene could be amplified from ∆mrpigG using primers hph F-hph R, while nothing was obtained from M. ruber M7. Meanwhile, amplicons of M. ruber M7 (2.28 kb) and ∆mrpigG (3.89 kb) differed in size when primers pigG d5F-pigG d3R annealing to homologous arms (5 flanking and 3 flanking regions) were used. J. Fungi 2020, 6, x FOR PEER REVIEW 6 of 15 Three ΔmrpigG::mrpigG strains with G418 resistance were obtained. PCR analysis of one of the ΔmrpigG::mrpigG strains is shown in Figure 2c. A 0.5-kb product was amplified when the genomic DNA of ΔmrpigG::mrpigG was used as template with primers pigG F2-pigG R2, whereas no DNA band was amplified using the genome of ΔmrpigG. A 1.22-kb fragment of the neo gene could be amplified from ΔmrpigG::mrpigG using primers G418 F-G418 R, whereas nothing was obtained from ΔmrpigG. A 2.14-kb product of the hph gene could be amplified from ΔmrpigG using primers hphF- Three ∆mrpigG::mrpigG strains with G418 resistance were obtained. PCR analysis of one of the ∆mrpigG::mrpigG strains is shown in Figure 2c. A 0.5-kb product was amplified when the genomic DNA of ∆mrpigG::mrpigG was used as template with primers pigG F2-pigG R2, whereas no DNA band was amplified using the genome of ∆mrpigG. A 1.22-kb fragment of the neo gene could be amplified from ∆mrpigG::mrpigG using primers G418 F-G418 R, whereas nothing was obtained from ∆mrpigG. A 2.14-kb product of the hph gene could be amplified from ∆mrpigG using primers hphF-hphR, whereas nothing was obtained from ∆mrpigG::mrpigG. The neo fragment replaced the hph gene, which might reveal that it was a successful homologous recombination event. Meanwhile, amplicons of ∆mrpigG::mrpigG (3.9 kb) and ∆mrpigG (4.3 kb) differed in size when primers pigG d5F-pigG d3R, annealing to homologous arms, were used. Seven M7::PtrpC-mrpigG strains with G418 resistance were obtained and checked by PCR analysis. As shown in Figure 3c, amplicons of M. ruber M7 and M7::PtrpC-mrpigG were equal in size when primers pigG F2-pigG R2 were used, and the size was 0.5 kb. A 1.22-kb fragment of the neo gene and a 0.37-kb product of the PtrpC gene could be amplified from M7::PtrpC-mrpigG using primers G418 F-G418 R and PtrpC F-PtrpR, whereas nothing was obtained from M7, respectively. Meanwhile, amplicons of M7::PtrpC-mrpigG (4.13 kb) and M. ruber M7 (2.56 kb) differed in size when primers pigG d5F-pigG d3R annealing to homologous arms were used, which proved that there was a single copy of the PtrpC-mrpigG overexpression construct integrated in M7::PtrpC-mrpigG. Seven M7::PtrpC-mrpigG strains with G418 resistance were obtained and checked by PCR analysis. As shown in Figure 3c, amplicons of M. ruber M7 and M7::PtrpC-mrpigG were equal in size when primers pigG F2-pigG R2 were used, and the size was 0.5 kb. A 1.22-kb fragment of the neo gene and a 0.37-kb product of the PtrpC gene could be amplified from M7::PtrpC-mrpigG using primers G418 F-G418 R and PtrpC F-PtrpR, whereas nothing was obtained from M7, respectively. Meanwhile, amplicons of M7::PtrpC-mrpigG (4.13 kb) and M. ruber M7 (2.56 kb) differed in size when primers pigG d5F-pigG d3R annealing to homologous arms were used, which proved that there was a single copy of the PtrpC-mrpigG overexpression construct integrated in M7::PtrpC-mrpigG. The transcription levels of the ΔmrpigG gene in the four strains-ΔmrpigG, ΔmrpigG::mrpigG, M7::Ptrp C-ΔmrpigG and the wild-type strain M. ruber M7-were analyzed by RT-qPCR. As shown in Figure 4, ΔmrpigG was deficient in the expression of the mrpigG gene, which further verified the success of the gene knockout. The tendency of mrpigG expression in ΔmrpigG::mrpigG was similar with that of M. ruber M7, and the average level of mrpigG expression in M7::PtrpC-mrpigG was higher than that of M. ruber M7, in spite of the 3rd day. The transcription levels of the ∆mrpigG gene in the four strains-∆mrpigG, ∆mrpigG::mrpigG, M7::Ptrp C-∆mrpigG and the wild-type strain M. ruber M7-were analyzed by RT-qPCR. As shown in Figure 4, ∆mrpigG was deficient in the expression of the mrpigG gene, which further verified the success of the gene knockout. The tendency of mrpigG expression in ∆mrpigG::mrpigG was similar with that of M. ruber M7, and the average level of mrpigG expression in M7::PtrpC-mrpigG was higher than that of M. ruber M7, in spite of the 3rd day.     Previous studies [5,9] have demonstrated that M. ruber M7 can produce MPs and CIT, but no MK, so the yields of the eight main MPs (four yellow pigments: monasfloure A, monascine, monasfloure B and ankaflavin; two orange pigments: rubropunctatin and monascorubrin; and two red pigments: rubropunctamine and monascorubramine) ( Figure 6) and CIT in M. ruber M7 and its mutants were analyzed in this study to uncover the effect of mrpigG on SMs. As for the yellow pigment production, the levels of monasfloure A, monascine, monasfluore B and ankaflavin in ∆mrpigG were 30.7%, 40.1%, 67.2% and 94% of that in M. ruber M7, respectively (Figure 7a-d). For orange and red pigment production, the production of rubropunctatin (an orange pigment (OP)) and rubropunctamine (a red pigment (RP)) in ∆mrpigG, both of which contain a 5-carbon side chain, were 57.7% and 22.3% of those in M. ruber M7 (Figure 7e-f). On the other hand, monascorubrin (an OP) and monascorubramine (an RP) in ∆mrpigG, both of which have a 7-carbon side chain, were 1.15 and 2.55 times those of M. ruber M7 (Figure 7g-h).

M7
Previous studies [5,9] have demonstrated that M. ruber M7 can produce MPs and CIT, but no MK, so the yields of the eight main MPs (four yellow pigments: monasfloure A, monascine, monasfloure B and ankaflavin; two orange pigments: rubropunctatin and monascorubrin; and two red pigments: rubropunctamine and monascorubramine) ( Figure 6) and CIT in M. ruber M7 and its mutants were analyzed in this study to uncover the effect of mrpigG on SMs. As for the yellow pigment production, the levels of monasfloure A, monascine, monasfluore B and ankaflavin in ΔmrpigG were 30.7%, 40.1%, 67.2% and 94% of that in M. ruber M7, respectively (Figure 7a-d). For orange and red pigment production, the production of rubropunctatin (an orange pigment (OP)) and rubropunctamine (a red pigment (RP)) in ΔmrpigG, both of which contain a 5-carbon side chain, were 57.7% and 22.3% of those in M. ruber M7 (Figure 7e-f). On the other hand, monascorubrin (an OP) and monascorubramine (an RP) in ΔmrpigG, both of which have a 7-carbon side chain, were 1.15 and 2.55 times those of M. ruber M7 (Figure 7g-h).
As shown in Figure 7i, the tendency of all strains' extracellular citrinin production was always increased from the 3rd day to the 11th day of cultivation in PDA media. During the early stage (3-7 days) the CIT production in M. ruber M7 was only 22.8-72.9% that of ΔmrpigG, whereas the other two mutants possessed similar yields to those of ΔmrpigG. At the end of the 11 days of fermentation, extracellular citrinin concentration of the four strains were similar to those of the wild-type strain M. ruber M7, at 1.88 ± 0.25 µg/mg citrinin.

Discussion
MPs with a common azaphilone skeleton are a large group of secondary metabolites produced via the polyketide pathway by filamentous fungi, mainly by Monascus spp.and Penicillium spp. [1,17]. MPs have been used in the food coloring, pharmaceutical, cosmetics, textile, printing and dyeing industries [3,18]. Recently, researchers proposed a MP biosynthetic pathway, in order to explore the generation of the bewildering diversity and complexity of MPs. To date, the functions of many genes in the MP gene clusters have been well investigated in M. ruber M7 and M. purpureus KACC42430, but there has been limited research on a few related genes, such as mpigG, mpigH and mpigI [8].
In current study, the generation of mrpigG mutants by homologous recombination, as shown in Figures 5 and 7, demonstrated that mrpigG was a member of the MP gene cluster. Searched with the Pfam 33.1 program, the data showed that mrpigG belonged to the serine hydrolase family, which is a large, ubiquitous family of enzymes grouped based on their ability to perform hydrolysis reactions on a range of biological substrates, such as ester, thioester, amide and epoxide bonds in small molecules, peptides or proteins [19][20][21]. The majority of serine hydrolases (>60%) adopt an α,β- As shown in Figure 7i, the tendency of all strains' extracellular citrinin production was always increased from the 3rd day to the 11th day of cultivation in PDA media. During the early stage (3-7 days) the CIT production in M. ruber M7 was only 22.8-72.9% that of ∆mrpigG, whereas the other two mutants possessed similar yields to those of ∆mrpigG. At the end of the 11 days of fermentation, extracellular citrinin concentration of the four strains were similar to those of the wild-type strain M. ruber M7, at 1.88 ± 0.25 µg/mg citrinin.

Discussion
MPs with a common azaphilone skeleton are a large group of secondary metabolites produced via the polyketide pathway by filamentous fungi, mainly by Monascus spp.and Penicillium spp. [1,17]. MPs have been used in the food coloring, pharmaceutical, cosmetics, textile, printing and dyeing industries [3,18]. Recently, researchers proposed a MP biosynthetic pathway, in order to explore the generation of the bewildering diversity and complexity of MPs. To date, the functions of many genes in the MP gene clusters have been well investigated in M. ruber M7 and M. purpureus KACC42430, but there has been limited research on a few related genes, such as mpigG, mpigH and mpigI [8].
In current study, the generation of mrpigG mutants by homologous recombination, as shown in Figures 5 and 7, demonstrated that mrpigG was a member of the MP gene cluster. Searched with the Pfam 33.1 program, the data showed that mrpigG belonged to the serine hydrolase family, which is a large, ubiquitous family of enzymes grouped based on their ability to perform hydrolysis reactions on a range of biological substrates, such as ester, thioester, amide and epoxide bonds in small molecules, peptides or proteins [19][20][21]. The majority of serine hydrolases (>60%) adopt an α,β-hydrolase fold and employ a Ser-His-Asp catalytic triad [21], but more than half of the serine hydrolases (>120 enzymes) remain poorly annotated, with no described physiological function or identified substrates [19,22].
In this study, the results showed that there was a 10-70% decrease in the four typical yellow pigments by mrpigG deletion. Simultaneously, the yields of typical orange pigments (OPs) and red pigments (RPs) revealed a completely different accumulation. In detail, compared with M. ruber M7, the yields of rubropunctatin (an OP) and its derivative rubropunctamine (an RP) produced by ∆mrpigG reduced to 57.7% and 22.3%, respectively. On the contrary, the production of monascorubrin (an OP) and its derivative monascorubramine (an RP) increase by 0.15-fold and 1.55-fold, respectively. These two OPs and RPs separately possessed the same azaphilone skeleton. In the same vein, rubropunctatin (OP) and rubropunctamine (RP) contained the −C 5 H 11 side chain, wheres the monascorubrin (OP) and monascorubramine (RP) owned the −C 7 H 15 side chain. The serine hydrolase encoded by mpigG specifically catalyzed the synthetic compounds with the −C 5 H 11 side chain, and might show substrate specificity to a certain extent. Navia-Paldanius et al. delineated the substrate preferences of three serine hydrolases by analyzing accumulation of fatty acid esters in cell lysates individually overexpressing each of the enzymes in assay mixes containing different chain length and saturation acylglycerols [23]. Ritchie et al. elaborated that the human type II thioesterase TE2 (containing a Ser-Asp-His catalytic triad) preferred an engineered human acyl-ACP substrate releasing short chain fatty acids from full-length fatty acid synthase (FASN) during turnover. The ability of TE2 to release fatty acids from FASN was involved in polyketide and non-ribosomal peptide synthase synthases [24].
Taking OP and RP production into account, the function of mrpigG in the biosynthesis pathway of MPs was proposed, as shown in Figure 8. In the proposed pathway, the β-ketooctanoic or β-ketodecanoic acid moieties used for this acylation step (P2) are produced by a dedicated two-subunit fatty acid synthetase (MrPigJ and MrPigK in M. ruber M7) encoded in the MP gene clusters. The β-keto fatty acid, along with ACP, specially hydrolyzes by means of MrPigG, then the hydrolysates are transferred to the acyltransferase (MrPigD). The C-4 alcohol of the benzopyran in P1 is then acylated with a medium chain β-keto fatty acid by MrPigD to yield the putative intermediate P2, which has been characterized as a precursor of MPs [8,9,25]. MrPigG may act as a metabolic serine hydrolase, like type II TE proteins, promoting the release of the short chain compounds from the thioester moiety on ACP [10,26].
J. Fungi 2020, 6, x FOR PEER REVIEW 13 of 15 hydrolase fold and employ a Ser-His-Asp catalytic triad [21], but more than half of the serine hydrolases (>120 enzymes) remain poorly annotated, with no described physiological function or identified substrates [19,22]. In this study, the results showed that there was a 10-70% decrease in the four typical yellow pigments by mrpigG deletion. Simultaneously, the yields of typical orange pigments (OPs) and red pigments (RPs) revealed a completely different accumulation. In detail, compared with M. ruber M7, the yields of rubropunctatin (an OP) and its derivative rubropunctamine (an RP) produced by ΔmrpigG reduced to 57.7% and 22.3%, respectively. On the contrary, the production of monascorubrin (an OP) and its derivative monascorubramine (an RP) increase by 0.15-fold and 1.55-fold, respectively. These two OPs and RPs separately possessed the same azaphilone skeleton. In the same vein, rubropunctatin (OP) and rubropunctamine (RP) contained the −C5H11 side chain, wheres the monascorubrin (OP) and monascorubramine (RP) owned the −C7H15 side chain. The serine hydrolase encoded by mpigG specifically catalyzed the synthetic compounds with the −C5H11 side chain, and might show substrate specificity to a certain extent. Navia-Paldanius et al. delineated the substrate preferences of three serine hydrolases by analyzing accumulation of fatty acid esters in cell lysates individually overexpressing each of the enzymes in assay mixes containing different chain length and saturation acylglycerols [23]. Ritchie et al. elaborated that the human type II thioesterase TE2 (containing a Ser-Asp-His catalytic triad) preferred an engineered human acyl-ACP substrate releasing short chain fatty acids from full-length fatty acid synthase (FASN) during turnover. The ability of TE2 to release fatty acids from FASN was involved in polyketide and non-ribosomal peptide synthase synthases [24].
Taking OP and RP production into account, the function of mrpigG in the biosynthesis pathway of MPs was proposed, as shown in Figure 8. In the proposed pathway, the β-ketooctanoic or βketodecanoic acid moieties used for this acylation step (P2) are produced by a dedicated two-subunit fatty acid synthetase (MrPigJ and MrPigK in M. ruber M7) encoded in the MP gene clusters. The βketo fatty acid, along with ACP, specially hydrolyzes by means of MrPigG, then the hydrolysates are transferred to the acyltransferase (MrPigD). The C-4 alcohol of the benzopyran in P1 is then acylated with a medium chain β-keto fatty acid by MrPigD to yield the putative intermediate P2, which has been characterized as a precursor of MPs [8,9,25]. MrPigG may act as a metabolic serine hydrolase, like type II TE proteins, promoting the release of the short chain compounds from the thioester moiety on ACP [10,26]. In conclusion, the mrpigG gene pertains to the MP biosynthetic gene cluster of M. ruber and plays an unignorable role in the biosynthesis of MPs. The disruption, complementation and overexpression of mrpigG had very little effect on the biomass, citrinin production and morphology of the strains. Compared with M. ruber M7, ΔmrpigG equally yielded eight typical pigments, but the production of four yellow pigments was partially restrained, and the synthesis of orange and red pigments In conclusion, the mrpigG gene pertains to the MP biosynthetic gene cluster of M. ruber and plays an unignorable role in the biosynthesis of MPs. The disruption, complementation and overexpression of mrpigG had very little effect on the biomass, citrinin production and morphology of the strains. Compared with M. ruber M7, ∆mrpigG equally yielded eight typical pigments, but the production of four yellow pigments was partially restrained, and the synthesis of orange and red pigments depended on side chain specificity. The content of rubropunctatin (OP) and its derivative rubropunctamine (RP), both of which have a five-carbon side chain, were 57.7% and 22.3% of those in M. ruber M7. On the other hand, monascorubrin (OP) and its derivative monascorubramine (RP), both of which have a seven-carbon side chain, were 1.15 and 2.55 times of those found in M. ruber M7. This work will make some contribution to the study of the biosynthesis pathway of MPs in M. ruber.
Author Contributions: L.L. designed and carried out the present research work, conducted experiments, analyzed the data and wrote the present manuscript. F.C. provided a place in the laboratory and gave access to the lab facilities for experimentation and funds for the present work. All authors have read and agreed to the published version of the manuscript.