THE HERPES-SIMPLEX VIRUS-1 U(L)15 GENE ENCODES 2 PROTEINS AND IS REQUIRED FOR CLEAVAGE OF GENOMIC VIRAL-DNA

Previous studies have shown that a ts mutant [herpes simplex virus 1(mP)ts66.4] in the U(L)15 gene fails to package viral DNA into capsids (A. P. W. Poon and B. Roizman, J. Virol. 67:4497-4503, 1993) and that although the intron separating the first and second exons of the U(L)15 gene contains U(L)16 and U(L)17 open reading frames, replacement of the first exon with a cDNA copy of the entire gene does not affect viral replication (J. D. Baines, and B. Roizman, J. Virol. 66:5621-5626, 1992). We report that (i) a polyclonal rabbit antiserum generated against a chimeric protein consisting of the bacterial maltose-binding protein fused in frame to the majority of sequences contained in the second exon of the U(L)15 gene reacted with two proteins with M(r) of 35,000 and 75,000, respectively, in cells infected with a virus containing the authentic gene yielding a spliced mRNA or with a virus in which the authentic U(L)15 gene was replaced with a cDNA copy. (ii) Insertion of 20 additional codons into the C terminus of U(L)15 exon II caused a reduction in the electrophoretic mobility of both the apparently 35,000- and 75,000-M(r) proteins, unambiguously demonstrating that both share the carboxyl terminus of the U(L)15 exon II. (iii) Accumulation of the 35,000-M(r) protein was reduced in cells infected and maintained in the presence of phosphonoacetate, an inhibitor of viral DNA synthesis. (iv) The U(L)15 proteins were localized in the perinuclear space at 6 h after infection and largely in the nucleus at 12 h after infection. (v) Viral DNA accumulating in cells infected with herpes simplex virus 1(mP)ts66.4 and maintained at the nonpermissive temperature was in an endless (concatemeric) form, and therefore U(L)15 is required for the cleavage of mature, unit-length molecules for packaging into capsids.