Six cell fusion-causing syn mutants were isolated from the KOS (syn-101 to syn-106) and three from the HFEM (syn-107 to syn 109) strains of herpes simplex virus type 1 (HSV-1). The mutants were studied by complementation and recombination with syn-20 (a syncytial mutant of KOS) and ts-B5 (a syncytial mutant of HFEM). Some studies also employed MP, a syncytium-inducing strain isolated from the non-syncytial parent, mP. Complementation and recombination of syn-20 and ts-B5 indicated that these two mutants were altered in two different virus genes. The recombination frequency between syn-20 and ts-B5 was very similar to that observed between MP and ts-B5, indicating that syn-20 and MP may represent alterations in the same virus gene. syn-101, syn-103, syn-104 and syn-105 were tentatively assigned to the syn-20 complementation group, while syn-107 and syn-109 were tentatively assigned to the ts-B5 complementation group. syn.106 and syn-108 were excluded from the ts-B5 group. syn-102 could not be excluded from either complementation group. syn-101 induced markedly less fusion at 38°C relative to 34°C. At 34° C the patterns of syn-101-infected cell peptides and glycopeptides, examined by SDS-gel electrophoresis, were normal, but at 38° C the amount of glycopeptide gC was particularly reduced. syn-102 produced decreased amounts of glycoproteins, and a non-glycosylated peptide, probably ICP6, was absent from extracts infected with syn-106.
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