Herpes simplex virus type 1 glycoprotein B (gB) is essential for virus entry, an event involving fusion of the virus envelope with the cell surface membrane, and virus-induced cell-cell fusion, resulting in polykaryocyte, or syncytium, formation. The experiments described in this report employed a random mutagenesis strategy to develop a more complete genetic map of mutations resulting in the syn mutant phenotype. The results indicate that syn mutations occur within two essential and highly

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... erved hydrophilic, ca-helical regions of the gB cytoplasmic domain. Region I is immediately proximal to the transmembrane domain and includes residues R796 to E816/817. Region H is localized centrally in the cytoplasmic domain and includes residues A855 and R858. Positively charged residues were particularly affected in both regions, suggesting that charge interactions may be required to suppress the syn mutant phehotype. No syn mutations were identified within the transmembrane domain. A virus containing a rate of entry (roe) mutation at residue A851, either within or