Map location of the pcbA mutation and physiology of the mutant.

Many temperature-resistant revertants of a polA1 polB polCts (HS432) strain are PolI+ (by either suppression of the polA1 amber allele or intragenic reversion) but remain polCts (contain a temperature-sensitive DNA polymerase III). It appears that DNA replication in such temperature-resistant revertants depends on an extragenic mutation, pcbA, already present in the parent strain and not linked to any of the DNA polymerase loci. This allele allows DNA replication dependent on DNA polymerase I and bypasses a temperature-sensitive DNA polymerase III (polC bypass), so that reversion to PolI+ makes the strain temperature resistant. This pathway of DNA replication also supports phage and plasmid DNA replication. At restrictive temperature, these mutants display a normal response to UV irradiation but show increased sensitivity to the alkylating agent methyl methanesulfonate. We have located pcbA linked to dnaA.