Study of the orientation of DNA in filamentous bacteriophage
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The single-stranded DNA of filamentous phage is oriented in the mature virion with a hairpin loop called mos at its leading end. mos has been shown to initiate morpohogenesis and to enhance phage yield. To better understand how and when the phage imposes this orientation on its DNA, I used a novel electron microscopic method to investigate the orientation of several mutant strains of the virus, as well as the pV/ssDNA complex--an intermediate protein/DNA structure formed during the phage life cycle. I found that the pV/ssDNA complex has mos at one end, as does the mature phage. This same orientation was evident in mutants that do not form a pV/ssDNA complex, and also in polyphages, that package many copies of the DNA molecule, with all but the first being packaged without going through initiation. Further, I found no dominant orientation in the absence of mos, but there was orientation imposed by the complement of mos. From my results, we postulate that orientation is mediated by mos in two stages: it imposes the axis of the DNA during formation of the complex, and it imposes the direction at initiation of virion assembly. The second step is where mos acts to enhance phage yield, according to my theory. Evidently, the two functions of mos are directed by two different properties of mos.
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