Development of bacteriophage Ha2, a phiX174 derivative, in Escherichia coli strains carrying a thermosensitive mutation in the dnaG gene.

Ha2 is a derivative of HaHb, a phiX174 mutant able to grow on several Escherichia coli K 12 strains which are insensitive to phiX174 wild type. Ha2 was isolated after nitrous acid mutagenesis and selected as being able to give large plaques in equal number at 42 degrees C as well as 30 degrees C when plated on Escherichia coli C and CR34. Hosts carrying a thermosensitive mutation at the dnaG locus are unable to support the replication and maturation of Ha2 at the restrictive temperature. The first steps of Ha2 development: attachment, eclipse and penetration, are not affected by the dnaG mutation. The viral DNA penetrates the cell as single-strands, which are probably attached to a viral protein component. The following steps of viral replication depend on the presence of an active dnaG gene product: no formation of parental RF; no synthesis and replication of progeny RF were observed in the mutant strains at 42 degrees C. The synthesis of viral single-stranded DNA as well as phage formation are lowered after a shift to the restrictive conditions, suggesting that the dnaG gene product is also involved at these stages.