We have used a partially reconstituted replication system consisting of T7 DNA polymerase and T7 gene 4 protein to examine the effect of benzo[a]pyrene (B[a]P) adducts on DNA synthesis and gene 4 protein activities. The gene 4 protein is required for T7 DNA replication because of its ability to act as both a primase and helicase. We show here that total synthesis decreases as the level of adducts per molecule of DNA increases, suggesting that the B[a]P adducts are blocking an aspect of the replication process. Polyacrylamide gels indicate that a shorter DNA product is produced on modified templates and this is confirmed by determining the average chain lengths from the ratio of chain initiations to chain elongation. Gene 4 protein primed synthesis reactions display a greater sensitivity to the presence of B[a]P adducts than do oligonucleotide-primed reactions. By challenging synthesis on oligonucleotide-primed B[a]P-modified DNA with unmodified DNA, we present evidence that the T7 DNA polymerase freely dissociates after encountering an adduct. Prior studies [Brown, W. C., & Romano, L. J. (1989) J. Biol. Chem. 264, 6748-6754] have shown that the gene 4 protein alone does not dissociate from the template during translocation upon encountering an adduct. However, when gene 4 protein primed DNA synthesis is challenged, we observe an increase in synthesis but to lesser extent than observed on oligonucleotide-primed synthesis.(ABSTRACT TRUNCATED AT 250 WORDS)