THE U(L)21 GENE-PRODUCTS OF HERPES-SIMPLEX VIRUS-1 ARE DISPENSABLE FOR GROWTH IN CULTURED-CELLS

A viral deletion mutant (Delta U(L)21) that lacked the sequences encoding 484 of the predicted first 535 amino acids of the U(L)21 open reading frame,vas genetically engineered and studied with respect to its phenotype in cells in culture. We report the following. (i) The replication of Delta U(L)21 was identical to that of the parent herpes simplex virus 1 (HSV-1) strain F in Vero cells, but the yields were three- to fivefold lower than those of the parent virus in human embryonic lung cells. (ii) To characterize the U(L)21 protein, we immunized rabbits against a purified bacterial fusion protein consisting of glutathione S-transferase fused to the majority of the coding domain of the U(L)21 gene. Rabbit antiserum directed against the fusion protein recognized a broad band with an apparent M(r) of 62,000 to 64,000 in lysates of cells infected with HSV-1 strain F and in virions purified from the infected cell cytoplasm. This band was absent from lysates of mock-infected cells or cells infected with the Delta U(L)21 virus. The band was significantly reduced in intensity in lysates of cells infected in the presence of phosphonoacetic acid, indicating that it is expressed as a late (gamma 1) gene. (iii) Immunofluorescence studies localized the U(L)21 antigen primarily in brightly staining granules in the cytoplasms of infected cells. Taken together, the data indicate that the U(L)21 protein is a virion component dispensable for all aspects of replication of HSV-1 in the cells tested. The electrophoretic mobility of the U(L)21 protein suggests that it is extensively modified posttranslationally.