Identical 371-base-pair deletion mutations in the LAT genes of herpes simplex virus type 1 McKrae and 17syn+ result in different in vivo reactivation phenotypes

The herpes simplex virus type 1 (HSV-1) LAT gene is the only viral gene abundantly transcribed during latency. LAT null mutants created with strains McKrae and 17syn+ are impaired for both in vivo spontaneous and in vivo-induced reactivation. Thus, LAT is essential for efficient in vivo-induced and spontaneous reactivation. Different investigators have studied two LAT mutants containing a StyI-StyI region deletion corresponding to LAT nucleotides 76 to 447. One mutant, dLAT371 (parent strain, McKrae), had parental high frequencies of spontaneous reactivation. In vivo-induced reactivation was not examined. The other mutant, 17DeltaSty (parent strain, 17syn+), had parental frequencies of in vitro reactivation following cocultivation of explanted ganglia but reduced frequencies of in vivo-induced reactivation. Spontaneous reactivation frequency was not reported for 17DeltaSty. These combined results suggested the possibility that in vivo spontaneous reactivation and in vivo-induced reactivation may map to different regions within the LAT domain. We now report that dLAT371 has in vivo-induced reactivation frequencies of the parent and that 17DeltaSty has reduced frequencies of in vivo spontaneous reactivation. Thus, dLAT371 demonstrated the parental phenotype for both in vivo spontaneous and -induced reactivation while the apparently identical 17DeltaSty was impaired for both in vivo spontaneous and -induced reactivation. These results suggest that one or more differences between the genetic backgrounds of McKrae and 17syn+ result in different in vivo reactivation phenotypes of otherwise identical deletion mutations and that McKrae may have compensating sequences sufficient to overcome the loss of the StyI-StyI region of the LAT transcript.