Role of a Mutation in Human Cytomegalovirus Gene UL104 in Resistance to Benzimidazole Ribonucleosides

The benzimidazole D-ribonucleosides TCRB and BDCRB are potent and selective inhibitors of human cytomegalovirus (HCMV) replication. Two HCMV strains resistant to these compounds were selected and had resistance mutations in genes UL89 and UL56. Proteins encoded by these two genes are the two subunits of the HCMV "terminase" and are necessary for cleavage and packaging of viral genomic DNA, a process inhibited by TCRB and BDCRB. We now report that both strains also have a previously unidentified mutation in UL104, the HCMV portal protein. This mutation, which results in L21F substitution, was introduced into the genome of wild-type HCMV by utilizing a recently cloned genome of HCMV as a bacterial artificial chromosome. The virus with this mutation alone was not resistant to BDCRB, suggesting that this site is not involved in binding benzimidazole nucleosides. As in previous proposals for mutations in UL104 of murine cytomegalovirus and HCMV strains resistant to BAY 38-4766, we hypothesize that this mutation could compensate for conformational changes in mutant UL89 and UL56 proteins, since the HCMV terminase is likely to interact with the portal protein during cleavage and packaging of genomic DNA.