dnaQ-encoded epsilon subunit of DNA polymerase III, possesses 3',5' exonuclease (proofreading) activity, and is a fidelity factor of polymerase III holoenzyme. It is assumed that during SOS-induced mutagenesis, UmuD', UmuC, and RecA may suppress DnaQ proofreading activity, and allow for translesional DNA synthesis at the cost of fidelity of replication. In this report SOS-dependent, MMS-induced mutagenesis and DNA repair were tested in E. coli dnaQ49 strains. Bacteria were transformed with various pDNAs harboring compilation of the umuD(D')C genes, and the influence of plasmids on mutagenesis (argE3-->Arg+) and DNA repair was tested. DNA damage and repair were tested in plasmid DNA grown in MMS-treated bacteria and isolated either immediately after MMS treatment, or after starving the cells (MFD conditions) for 30 and 60 min, then nicking activity of Fpg protein on plasmid DNAs was analyzed. It has been found that (i) repair of MMS-induced lesions depends on umuD'C, umuD' (and to much less degree, on umuDC) genes encoded in pDNA; (ii) MMS-induced mutations, in contrast to DNA repair, are highest in the cells transformed with pDNA harboring umuDC, and lowest or zero in cells with plasmids harboring umuD'C. It is postulated that UmuD'C or UmuD' proteins play a role in the repair of damaged DNA and/or in maintenance of DNA integrity. The kinetics of these processes (perhaps due to introducing too many of the lesions) seems to be different in E. coli dnaQ+ and dnaQ cells, and probably this is a reason that (iii) MMS-induced mutations in dnaQ49 strains are not subject to MFD.