ICP0 is a potent activator of herpes simplex virus type 1 gene expression in transient assays and in productive infection. A role for ICP0 in reactivation from latency in vivo has also been suggested on the basis of the observation that viruses with mutations in both copies of the diploid gene for ICP0 reactivate less efficiently than wild-type virus. Because the ICP0 gene is contained entirely within the coding sequences for the latency-associated transcripts (LATs), ICP0 mutants also contain mutations in LAT coding sequences. This overlap raises the question of whether mutations in ICP0 or the LATs, which have also been implicated in reactivation, are responsible for the reduced reactivation frequencies characteristic of ICP0 mutants. Two approaches were taken to examine more definitively the role of ICP0 in the establishment and reactivation of latency. First, a series of ICP0 nonsense, insertion, and deletion mutant viruses that exhibit graded levels of ICP0-specific transactivating activity were tested for parameters of the establishment and reactivation of latency in a mouse ocular model. Although these mutants are ICP0 LAT double mutants, all nonsense mutants induced the synthesis of near-wild-type levels of the 2-kb LAT, demonstrating that the nonsense linker did not disrupt the synthesis of this LAT species. All mutants replicated less efficiently than the wild-type virus in mouse eyes and ganglia during the acute phase of infection. The replication efficiencies of the mutants at these sites corresponded well with the ICP0 transactivating activities of individual mutant peptides in transient expression assays. All mutants exhibited reduced reactivation frequencies relative to those of wild-type virus, and reactivation frequencies, like replication efficiencies in eyes and ganglia, correlated well with the level of ICP0 transactivating activity exhibited by individual mutant peptides. The amount of DNA of the different mutants varied in latently infected ganglia, as demonstrated by polymerase chain reaction analysis. No correlation was evident between reactivation frequencies and the levels of viral DNA in latently infected ganglia. Thus, replication and reactivation efficiencies of ICP0 mutant viruses correlated well with the transactivating efficiency of the corresponding mutant peptides. In a second approach to examining the role of ICP0 in latency, a single copy of the wild-type gene for ICP0 was inserted into the genome of an ICP0- LAT- double mutant, 7134, which exhibits a marked impairment in its ability to replicate in the mouse eye and reactivate from latency.(ABSTRACT TRUNCATED AT 400 WORDS)