Cis-limited action of the gene-A product of bacteriophage phiX174 and the essential bacterial site (E. coli-electron microscopy-cis-acting protein-specifically-nicked RF).

Parental replicative-form (RF(*)) DNA of bacteriophage varphiX174 in a replication-deficient host cell (rep(3) (-)) exhibits two characteristic features that correlate the function of viral gene A with the initiation of viral DNA replication: a specific discontinuity in the viral strand of a constant number of RF molecules and elongation of the viral strand to yield replicative-intermediate DNA forms with single-stranded tails. At high multiplicities of infection, these initiation events are limited to an average of four specifically nicked RFII molecules per cell. The limiting factor from the host cell may be related (or identical) to the essential bacterial sites known to limit the participation of parental genomes in RF replication. Double-infection experiments with wild-type phage and phage carrying an amber mutation in gene A show that the formation of gene A-specific RFII and RI is cis-limited to only the wild-type DNA. These results provide a basis at the DNA level for the known asymmetric complementation of gene A.