Herpes simplex virus type 1 encodes its own DNA polymerase (Pol), the product of the UL30 gene, and a polymerase accessory subunit, the product of the UL42 gene, both of which are required for viral DNA replication. Pol and the UL42 protein associate to form a heterodimeric complex (Pol/UL42) which is more active and has a higher processivity than the Pol catalytic subunit alone. The Pol/UL42 complex has been reconstituted by mixing together highly purified Pol and UL42 subunits obtained from recombinant baculovirus-infected cells. We have used polymerase activity on poly(dA):oligo(dT20), a template that the Pol subunit utilizes with low efficiency, to measure the formation of the Pol/UL42 complex. Our data indicate that the association constant for the Pol/UL42 complex is 1 x 10(8) M-1. Proteolytic digestions of UL42 were performed to determine whether structural domains of UL42 could be disclosed by differential amino acid accessibilities. The ability of these protease-resistant domains to form a functional complex with Pol was determined by measuring their ability to stimulate Pol activity on poly(dA):oligo(dT20). We have found that trypsin digestion of UL42 in the presence of DNA generates protease-resistant fragments of 28K and 8K which co-elute from a MonoQ column and are able to stimulate Pol activity on poly(dA):oligo(dT20). Complex formation of the 28K and 8K tryptic fragments with Pol was also shown by their co-immunoprecipitation with antibody to Pol. It was determined that the 28K fragment of UL42 comprised amino acids 1 to 245 or 1 to 254 of UL42, whereas the 8K fragment started at amino acid 255. Thus, controlled proteolysis of UL42 revealed two closely contiguous structural domains that retained the ability to complex with Pol and stimulate Pol activity.