The synthesis of bacteriophage G4 DNA was examined in temperature-sensitive dna mutants under permissive and nonpermissive conditions. The infecting single-stranded G4 DNA was converted to the parental replicative form (RF) at the nonpermissive temperature in infected cells containing a temperaturesensitive mutation in the dnaA, dnaB, dnaC, dnaE, or dnaG gene. The presence of 30 ,ug of chloramphenicol or 200 ,ig of rifampin per ml had no effect on parental RF synthesis in these mutants. Replication of G4 double-stranded RF DNA occurred at a normal rate in dnaAts cells at the nonpermissive temperature, but the rate was greatly reduced in cells containing a temperaturesensitive mutation in the dnaB, dnaC, dnaE, or dnaG gene. RF DNA replicated at normal rates in revertants of these dna temperature-sensitive host cells. The simplest interpretation of these observations is that none of the dna gene products tested is essential for the synthesis of the complementary DNA strand on the infecting single-stranded G4 DNA, whereas the dnaB, dnaC, dnaE (DNA polymerase III), and dnaG gene products are all essential for replication of the double-stranded G4 RF DNA. The alternate possibility that one or more of the gene products are actually essential for G4 parental RF synthesis, even though this synthesis is not defective in the mutant hosts, is also discussed.

The dnaH mutant strain HF4704S, isolated by Sakai et al. (1974), was examined for its effect on kX174 deoxyribonucleic acid (DNA) synthesis. It was found to carry two mutations affecting DNA synthesis. One mutation had no affect on OX174 DNA synthesis, but did affect the ability of the mutant cells to form colonies on agar medium at 410C, and caused host DNA synthesis to cease after 1 h at 410C. The mutant marker cotransduced with ilvD at a frequency of about 9%. It seems likely that this mutation is in the dnaA gene. The second mutation affected the ability of the mutant cells to form colonies on agar medium supplemented with only 2 ug of thymine per ml, and affected both host and OX174 DNA synthesis in medium supplemented with only 2 ug of thymine per ml. Both effects could be overcome by adding excess exogenous thymine. We were not able to unambiguously determine the map position of this mutant locus. Our data show that the DNA synthesis phenotype of the mutant strain HF4704S is governed by both of these mutations, neither of which directly affects the replication of 4X174 DNA.