The product of the UL12.5 gene of herpes simplex virus type 1 is not essential for lytic viral growth and is not specifically associated with capsids.

The herpes simplex virus type 1 UL12 gene encodes a pH-dependent deoxyribonuclease termed alkaline nuclease. An N-terminally truncated version of the UL12 gene, called UL12.5, was shown to be translated independently from a subgenic mRNA which shares its 3' terminus with the full-length UL12 mRNA. We showed previously that the UL12.5 gene product cannot compensate for the absence of the full-length UL12 gene product (R. Martinez, L. Shao, J. C. Bronstein, P. C. Weber, and S. K. Weller, 1996, Virology 215, 152-164); however, it was not known whether UL12.5 itself performs an essential function during lytic viral growth. In this article the initiation codon for the UL12.5 gene product was mapped and altered to create a gene no longer capable of producing UL12.5. This mutation was introduced into the viral genome to create a virus which was capable of producing full-length UL12 but not UL12.5. The growth properties of this virus indicate that UL12.5 is not essential for viral growth in culture. UL12.5 was previously reported to represent a capsid-associated form of alkaline nuclease (J. C. Bronstein, S. K. Weller, and P. C. Weber, 1997, J. Virol. 71, 3039-3047). Sucrose sedimentation analysis of capsids from cells infected with wild-type or mutant viruses indicates that both UL12 and UL12.5 are found in fractions from across the sucrose gradient which do not always correlate with the presence of viral capsids. Furthermore, UL12.5 is found in fractions across the gradient even in cells infected under conditions in which no capsids are formed. These results indicate that UL12.5 does not specifically associate with viral capsids. Taken together, these data indicate that UL12.5 is not likely to play an important role in lytic viral infection.