The mutated tegument protein UL7 attenuates the virulence of herpes simplex virus 1 by reducing the modulation of α-4 gene transcription.

Background: UL7, a tegument protein of Herpes Simplex Virus type I (HSV-1), is highly conserved in viral infection and proliferation and has an unknown mechanism of action.

Methods: A HSV-1 UL7 mutant (UL7-MU) was constructed using the CRISPR-cas9 system. The replication rate and plaque morphology were used to analyze the biological characteristics of the wild-type (WT), UL7-MU and MU-complemented P1 viruses. The virulence of the viruses was evaluated in mice. Real-time RT-qPCR and ChIP assays were used to determine the expression levels of relevant genes.

Results: The replication capacity of a recombinant virus (UL7-MU strain) was 10-fold lower than that of the WT strain. The neurovirulence and pathologic effect of the UL7-MU strain were attenuated in infected mice compared with the WT strain. In the latency model, the expression of latency-associated transcript (LAT) in the central nervous system (CNS) and trigeminal nerve was lower in UL7-MU-infected mice than in WT strain-infected mice. The transcription level of the immediate-early gene α-4 in UL7-MU-infected cells was reduced by approximately 2-fold compared with the clear transcriptional peak identified in WT strain-infected Vero cells within 7 h post-infection (p.i.).

Conclusion: By modulating the transcription of the α-4 gene, UL7 may be involved in transcriptional regulation through its interaction with the transcript complex structure of the viral genome during HSV-1 infection.