Abstract

Shortly after infection with bacteriophage PM2, host DNA synthesis is inhibited while synthesis of the supercoiled viral DNA begins at a low rate. This rate increases at about ten minutes after infection to reach a value that is thereafter maintained until lysis of the cells. Pulse-chase experiments suggest the existence of several precursors of supercoiled PM2 DNA. Two of them, one which sediments as nicked circular PM2 DNA (21 s) and another, appearing earlier in time, sedimenting 1.15 times faster (24 s), have been partially characterized. The 24 s component seems to contain linear single strands both longer and shorter than the strands of PM2 DNA. It has been purified, examined in the electron microscope and shown to consist of double-stranded rings of the circumference of PM2 DNA, with one double-stranded DNA branch of distinct length, but always shorter than PM2 DNA. From these characteristics the 24 s precursor seems to be a PM2 DNA molecule with a growing chain in accordance with the hypothesis that chain growth involves addition of nucleotides to the end of one parental strand while a second strand is initiated de novo. The pulse-labeled precursor which sediments like nicked circular PM2 DNA does not contain label in single-strand rings but only in open strands. Experiments based on transfer of the infected cell from heavy to light medium suggest a mechanism of replication which involves random selection of the viral DNA molecule to be duplicated from among these present in the cell.

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