DNA-POLYMERASE-I AND THE BYPASSING OF RECA DEPENDENCE OF CONSTITUTIVESTABLE DNA-REPLICATION IN ESCHERICHIA-COLI RNHA MUTANTS

In Escherichia coli rnhA mutants, several normally repressed origins ( oriK sites) of DNA replication are activated. The type of DNA replicat ion initiated from these origins, termed constitutive stable DNA repli cation, does not require DnaA protein or the oriC site, which are esse ntial for normal DNA replication. It requires active RecA protein. We previously found that the lexA71(Def)::Tn5 mutation can suppress this RecA requirement and postulated that the derepression of a LexA regulo n gene(s) leads to the activation of a bypass pathway, Rip (for RecA-i ndependent process). In this study, we isolated a miniTn10spc insertio n mutant that abolishes the ability of the lexA(Def) mutation to suppr ess the RecA requirement of constitutive stable DNA replication. Cloni ng and DNA sequencing analysis of the mutant revealed that the inserti on occurs at the 3' end of the coding region of the polA gene, which e ncodes DNA polymerase I. The mutant allele, designated polA25::miniTn1 0spc, is expected to abolish the polymerization activity but not the 5 '-->3' or 3'-->5' exonuclease activity. Thus, the Rip bypass pathway r equires active DNA polymerase I. Since the lethal combination of recA (Def) and polA25::miniTn10spc could be suppressed by derepression of t he LexA regulon only when DNA replication is driven by the oriC system , it was suggested that the bypass pathway has a specific requirement for DNA polymerase I at the initiation step in the absence of RecA. An accompanying paper (Y. Cao and T. Kogoma, J. Bacteriol. 175:7254-7259 , 1993) describes experiments to determine which activities of DNA pol ymerase I are required at the initiation step and discusses possible r oles for DNA polymerase in the Rip bypass pathway.