Replication of bacteriophage M13. XV. Location of the specific nick in M13 replicative form II accumulated in Escherichia coli polAex1.

M13 replicative form II (RFII) DNA was prepared from Escherichia coli RS5052 (polAex1) cells in the late stage of infection, and the DNA sequence at the discontinuity was examined. The data presented here suggest that the single discontinuity in the late stage of infection RFII maps at the same position as the gene II protein nicking site on fd RFI which was determined in vitro (Meyer et al., Nature (London) 278:365-367, 1979) and has a 5' terminal nucleotide sequence identical to that at the nick produced by gene II protein in vitro. The discontinuity in the in vivo RFII appears to be a single break in the phosphodiester backbone, leaving a 3' OH terminus. RFII molecules containing a gap, i.e., missing nucleotides at the site of discontinuity, were not detected.