Calf thymus proliferating cell nuclear antigen (PCNA) promoted DNA synthesis past cis-syn and trans-syn-I cyclobutane thymine dimers by calf thymus DNA polymerase delta (Pol delta) in vitro. Templates containing site-specific cis-syn and trans-syn-I thymine dimers were prepared via a combination of solid phase synthesis with photoproduct building blocks and DNA ligation. Extension of a 15-mer primer on the UV dimer-containing templates by Pol delta produced termination and bypass products in a dNTP and PCNA dependent manner. In the absence of PCNA and at dNTP concentrations varying between 1 and 100 microM, Pol delta could not bypass the cis-syn dimer and terminated elongation one nucleotide prior to the 3'-T of the dimer. DNA synthesis past the trans-syn-I dimer was even less efficient. In the presence of PCNA, termination occurred primarily one nucleotide prior to the 3'-T of both dimers at 1 microM dNTPs but opposite the 5'-T of the dimers at 100 microM dNTPs. In addition, under the latter conditions, bypass of the dimers was observed, to the extent of about 30% of the products for the cis-syn dimer and about 15% for the trans-syn-I dimer.