HSV-1 exonuclease UL12 interacts with host recombination factors and is essential for viral growth

Herpes simplex virus type 1 (HSV-1) belongs to a clinically significant and well studied group of viruses called the alphaherpesviruses. HSV-1 infects and replicates in the nucleus of host cells. Several lines of evidence suggest that HSV-1 uses a recombination-dependent mechanism to replicate its genome. HSV-1 encodes a 5\u274 3\u27 alkaline exonuclease (UL12) and a single strand binding protein (ICP8) which together mediate strand exchange in vitro. UL12 and ICP8 might function as a two-component recombinase analogous to the phage lambda Red α/β recombination system. The lambda exonuclease Red α and the single strand binding protein Red β mediate strand exchange and recombination, which are essential for phage DNA replication. The growth of lambda phage mutants defective for either of the red genes is severely compromised in RecA mutant E. coli (Enquist et al., 1973). This result suggests that the lambda phage can use either viral or cellular recombination proteins. In this thesis, our efforts to understand the role of viral and host recombination factors during HSV-1 replication led to the identification of host factors, the MRN complex and Ku70, components of the host recombination repair pathway to interact with the exonuclease component of the viral recombinase. We also report for the first time that UL12 is essential for viral growth in most human cells tested with the exception of HEK cells which are partially permissive for growth of a UL12 null mutant. We were also able to confirm previous reports that Vero and BHK cells were also partially permissive for the growth of the UL12 null virus. Our findings results indicate that the UL12 mutant virus has a severe defect in the production of DNA containing capsids. Phenotypic analysis of the UL12 null virus suggests an essential role for UL12 in the production of viral DNA that can be efficiently packaged. UL12 may play a role during viral DNA synthesis or at a post-replicative step necessary for processing the viral replication intermediates, or both, resulting in the generation of viral genomes that can be correctly cleaved and packaged to produce infectious virus.