In Escherichia coli rnhA mutants, several normally repressed origins (oriK sites) of DNA replication are activated. The type of DNA replication initiated from these origins, termed constitutive stable DNA replication, does not require DnaA protein or the oriC site, which are essential 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 regulon gene(s) leads to the activation of a bypass pathway, Rip (for RecA-independent process). In this study, we isolated a miniTn10spc insertion mutant that abolishes the ability of the lexA(Def) mutation to suppress the RecA requirement of constitutive stable DNA replication. Cloning and DNA sequencing analysis of the mutant revealed that the insertion occurs at the 3' end of the coding region of the polA gene, which encodes DNA polymerase I. The mutant allele, designated polA25::miniTn10spc, is expected to abolish the polymerization activity but not the 5'-->3' or 3'-->5' exonuclease activity. Thus, the Rip bypass pathway requires active DNA polymerase I. Since the lethal combination of recA(Def) and polA25::miniTn10spc could be suppressed by derepression of the 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 polymerase I are required at the initiation step and discusses possible roles for DNA polymerase in the Rip bypass pathway.