Replication of damaged DNA, also termed as translesion synthesis (TLS), involves specialized DNA polymerases that bypass DNA lesions. In Escherichia coli, although TLS can involve one or a combination of DNA polymerases depending on the nature of the lesion, it generally requires the Pol V DNA polymerase (formed by two SOS proteins, UmuD' and UmuC) and the RecA protein. In addition to being an essential component of translesion DNA synthesis, Pol V is also an antagonist of RecA-mediated recombination. We have recently isolated umuD' and umuC mutants on the basis of their increased capacity to inhibit homologous recombination. Despite the capacity of these mutants to form a Pol V complex and to interact with the RecA polymer, most of them exhibit a defect in TLS. Here, we further characterize the TLS activity of these Pol V mutants in vivo by measuring the extent of error-free and mutagenic bypass at a single (6-4)TT lesion located in double stranded plasmid DNA. TLS is markedly decreased in most Pol V mutants that we analyzed (8/9) with the exception of one UmuC mutant (F287L) that exhibits wild-type bypass activity. Somewhat unexpectedly, Pol V mutants that are partially deficient in TLS are more severely affected in mutagenic bypass compared to error-free synthesis. The defect in bypass activity of the Pol V mutant polymerases is discussed in light of the location of the respective mutations in the 3D structure of UmuD' and the DinB/UmuC homologous protein Dpo4 of Sulfolobus solfataricus.