The UL25 Gene Product of Herpes Simplex Virus type 1 is Involved in Uncoating of the Viral Genome

MRC Virology Unit, Institute of Virology, Church Street, Glasgow G11 5JR, United Kingdom

^* To whom correspondence should be addressed. Email: v.preston{at}mrcvu.gla.ac.uk.

	Abstract

Studies on the herpes simplex virus type 1 UL25 null mutant KUL25NS have shown that the capsid-associated UL25 protein is required at a late stage in the encapsidation of viral DNA. Our previous work on UL25 using the UL25 temperature sensitive (ts) mutant, ts1204, also implicated UL25 in a role at very early times in the virus growth cycle, possibly at the stage of penetration of the host cell. We have re-examined this mutant and discovered that it had an additional ts mutation elsewhere in the genome. The ts1204 UL25 mutation was transferred into wild type virus DNA and the UL25 mutant ts1249 was isolated and characterized to clarify the function of UL25 at the initial stages of virus infection. Indirect immunofluorescence assays and in situ hybridization analysis of virus-infected cells revealed that the mutant ts1249 was not impaired in penetration of the host cell but had an uncoating defect at the non-permissive temperature. When ts1249-infected cells were incubated initially at the permissive temperature to allow uncoating of the viral genome and subsequently transferred to the restrictive temperature, a DNA packaging defect was evident. The results suggested that ts1249, like KUL25NS, had a block at a late stage of DNA packaging and that the packaged genome was shorter than full length. Examination of ts1249 capsids produced at the non-permissive temperature revealed that, in comparison with wt capsids, they contained reduced amounts of UL25 protein, thereby providing a possible explanation for the failure of ts1249 to package full length viral DNA.