The RR1 gene of herpes simplex virus type 1 is uniquely trans activated by ICP0 during infection

As has been demonstrated for herpes simplex virus type 2, we show in this report that the herpes simplex virus type 1 ribonucleotide reductase large subunit (RR1) gene is trans activated in transient transfection assays by VP16 and ICP0 but not by ICP4. Deletion analysis demonstrated that responsiveness to induction to VP16 resides in an octamer/TAATGARAT sequence of the RR1 promoter and that the TATA box alone is sufficient to provide induction by ICP0. The induction of the RR1 gene by ICP0 but not by ICP4 suggested that it might be possible to identify the cis-acting element(s) responsive to ICP4 in an ICP4-inducible promoter. To this end, a series of chimeric promoters containing various portions of the regulatory sequences of the RR1 promoter and thymidine kinase (TK) promoter were constructed. The TK promoter is trans activated by both ICP0 and ICP4 in transient transfection assays and by ICP4 in infection. The data show that replacing the RR1 TATA region with the TK TATA region permits ICP4 inducibility even if the rest of the RR1 promoter elements remain intact. To test whether the RR1 gene is induced by ICP0 during infection, four mutant viruses were constructed, (i) TAATGARAT⁺ has the wild-type RR1 promoter driving chloramphenicol acetyltransferase (CAT) and the RR2 promoter driving the lacZ gene. The RR2 gene codes for the small subunit of the ribonucleotide reductase and is expressed as a β gene. (ii) TAATGARAT^- has a triple-base change in the octamer/TAATGARAT element which renders it unresponsive to VP16 trans activation, eliminating that portion of the activation of the RR1 gene. (iii) TAATGARAT^-Δα0 has a deletion of the α0 gene. (iv) TAATGARAT^-Δα4 has a deletion of the α4 gene. Infections were carried out in Vero cells at a multiplicity of infection of 10 per cell; cells were assayed for CAT and β-galactosidase (β-Gal) activities and for virus yields. The first two infections gave strong CAT and β-Gal activities and high yields of progeny virus. Infection with the third virus showed no CAT activity but did produce high levels of β-Gal activity and virus progeny. The fourth infection resulted in strong CAT activity but no β-Gal activity or progeny virus. The data demonstrated that the RR1 promoter was activated in the absence of ICP4 but not in the absence of ICP0 in these infections. We conclude that, in the absence of VP16 induction, ICP0 is the primary activator of the RR1 gene promoter in infection.