Herpes simplex virus type 1 ICP27 is an essential regulatory protein

The five immediate-early genes of herpes simplex virus are expressed during the initial stages of the infectious cycle, and certain immediate-early proteins have been shown to play a regulatory role in subsequent viral gene expression. Until recently, the functional properties of only one immediate-early protein, ICP4, had been examined in any detail, primarily because mutants had been isolated only in the gene for ICP4. We report herein the genetic and phenotypic characterization of four temperature-sensitive mutants of herpes simplex virus type 1 (tsY46, tsE5, tsE6, and tsLG4) that have begun to elucidate the function(s) of a second immediate-early protein, ICP27. The four mutants complemented each other inefficiently or not at all, indicating that they are defective in the same function. Marker rescue tests placed the mutations in tsY46 and tsE5 in sequences that encode the transcript for ICP27; the mutations in tsE6 and tsLG4 lie in or near these sequences. The ability of wild-type ICP27 expressed from a cloned gene to complement tsY46 and tsLG4 constitutes additional evidence that these mutants are defective in an ICP27-associated function. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of mutant-infected cell polypeptides showed that certain immediate-early (alpha) polypeptides were overproduced, whereas significant levels of early (beta) and drastically reduced levels of several late (gamma) proteins were synthesized at the nonpermissive temperature. Interestingly, the mutants were observed to form a spectrum with regard to their relative abilities to induce the expression of a number of polypeptides, especially those of the delayed-early (beta gamma) class. Consistent with their ability to induce expression of early polypeptides, all of the mutants induced the synthesis of substantial levels of viral DNA at the nonpermissive temperature. Taken together, the results of these studies demonstrate that ICP27 plays an essential regulatory function in virus replication, that this function is required after the onset of early gene expression and viral DNA synthesis, and that the inability of the mutants to induce the synthesis of late proteins is independent of viral DNA synthesis.