A “DNA crunching” linear motor mechanism that employs a grip-and-release transient spring like compression of B- to A-form DNA has been found in our previous studies. Our FRET measurements in vitro show a decrease in distance from TerL to portal during packaging; furthermore, there is a decrease in distance between closely positioned dye pairs in the Y-stem of translocating Y-DNA that conforms to B- and A- structure. In normal translocation into the prohead the TerL motor expels all B-form tightly binding YOYO-1 dye that cannot bind A-form. The TerL motor cannot package A-form dsRNA. Our work reported here shows that addition of helper B form DNA:DNA (D:D) 20mers allows increased packaging of heteroduplex A-form DNA:RNA 20mers (D:R), evidence for a B- to A-form spring motor pushing duplex nucleic acid. A-form DNA:RNA 25mers, 30mers, and 35mers alone are efficiently packaged into proheads by the TerL motor showing that a proposed hypothetical dehydration motor mechanism operating on duplex substrates does not provide the packaging motor force. Taken together with our previous studies showing TerL motor protein motion toward the portal during DNA packaging, our present studies of short D:D and D:R duplex nucleic acid substrates strongly supports our previous evidence that the protein motor pushes rather than pulls or dehydrates duplex substrates to provide the translocation into prohead packaging force.
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