A novel class of transcripts expressed with late kinetics in the absence of ICP4 spans the junction between the long and short segments of the herpes simplex virus type 1 genome.

A novel family of transcripts that span the junction between the long and short segments of the herpes simplex virus type 1 genome has been identified. These transcripts, designated L/S junction-spanning transcripts (L/STs), are synthesized in abundance in a variety of cells infected with mutant viruses defective in the gene for ICP4, the major transcriptional regulatory protein of the virus. Transcription of abundant 2.3- and 8.5-kb series of L/STs was shown to initiate within the same sequences as less abundant 4.2-, 7.3-, and > 9.5-kb transcripts by Northern (RNA) blot analysis. S1 nuclease analysis revealed a single 5' terminus 28 bp downstream of a TATA box and 6 bp downstream of a consensus ICP4 binding site. The location of the transcriptional start site indicates that the promoter of the L/STs likely corresponds to the bidirectional promoter described by Bohenzky et al. (R. A. Bohenzky, A. G. Papavassiliou, I. H. Gelman, and S. Silverstein, J. Virol. 67:632-642, 1993). The L/STs accumulate with late kinetics in ICP4 mutant-infected cells and are polyadenylated. Mutant viruses encoding forms of ICP4 unable to bind the consensus site, ATCGTC, exhibited abundant expression of the L/STs, whereas mutants encoding forms of ICP4 able to bind this site expressed no detectable L/STs, suggesting that ICP4 plays a critical role in repressing L/ST expression. Their synthesis in ICP4 mutant-infected cells is inhibited by the protein synthesis inhibitor cycloheximide, indicating that they are induced either by an immediate-early viral protein other than ICP4 or by a virus-induced cellular protein. Preliminary evidence indicates that the L/STs are not present in latently infected ganglia. The abundant expression of the L/STs with late kinetics only in the absence of functional ICP4 and the sensitivity of their synthesis to cycloheximide indicate that they are not members of any of the recognized kinetic classes of herpes simplex virus type 1 transcripts but constitute a new class of viral transcript.