Genetic loci that affect cell fusion in the 0.70 to 0.82 region of the HSV-1 genome were studied. A library of BamHI and EcoRI fragments from the KOS strain were cloned in pBR322 and pBR325, respectively. The EcoRI-B fragment and the BamHi-L fragment were used to generate working restriction maps of the region of the HSV-1 genome between 0.707 and 0.86. Then, a fine restriction map was made of the region between 0.724 and 0.81. These fragments were also used to construct overlapping subclones. Marker-rescue experiments employed these subcloned fragments along with intact mutant DNA from MP, syn-20, syn-102, syn-103, and syn-105. Because the syncytial marker is not selectable, total progeny plaques were screened using the presence of wild-type virus as an indicator of marker rescue. Several independent experiments showed that the mutations in all of these syn-mutants were rescued by KOS sequences between the map coordinates 0.732 and 0.745. MP and syn-20 mutations were further localized to map coordinates 0.737 to 0.74 or within 0.5 kilobases. This syn locus, which is cell-type dependent for the fusion phenotype, was named syn-1. A second syn mutation in MP was mapped at coordinates 0.745 to 0.761. This lesion, names syn-2, is also cell-type dependent for the fusion phenotype. The locus determining glycoprotein C (gC) production in strain MP was mapped by screening plaques for the presence of gC using indirect immunofluorescence. This function was localized to map coordinates 0.745 to 0.753. Based on the plaque morphology of several wild-type recombinants, the absence of gC appears to induce a small amount of cell fusion. The order of the three loci can be deduced, and is syn-1 syn-2 Cr. Previous work by others revealed that both syn-2 and Cr overlap the region to which the mRNA for ICP-27, an (alpha)-protein, was mapped. Mutations in (alpha)-proteins may, therefore, affect cell fusion, the production of gC, and possibly other glycoproteins. This last possibility was confirmed by glycoprotein gel profiles of several wild-type recombinants, all of which had taken up wild-type KOS sequences overlapping ICP-27 which cause alternations in the maturation of glycoprotein B (gB).
