Bypass synthesis by DNA polymerase II was studied using a synthetic 40-nucleotide-long gapped duplex DNA containing a site-specific abasic site analog, as a model system for mutagenesis associated with DNA lesions. Bypass synthesis involved a rapid polymerization step terminating opposite the nucleotide preceding the lesion, followed by a slow bypass step. Bypass was found to be dependent on polymerase and dNTP concentrations, on the DNA sequence context, and on the size of the gap. A side-by-side comparison of DNA polymerases I, II, and III core revealed the following. 1) Each of the three DNA polymerases bypassed the abasic site analog unassisted by other proteins. 2) In the presence of physiological-like salt conditions, only DNA polymerase II bypassed the lesion. 3) Bypass by each of the three DNA polymerases increased dramatically in the absence of proofreading. These results support a model (Tomer, G., Cohen-Fix, O. , O'Donnell, M., Goodman, M. and Livneh, Z. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 1376-1380) by which the RecA, UmuD, and UmuC proteins are accessory factors rather than being absolutely required for the core mutagenic bypass reaction in induced mutagenesis in Escherichia coli.