Positive and Negative Regulation at the Herpes Simplex Virus ICP4 and ICPO TAATGARAT Motifs

Abstract

The control of the ICP0 and ICP4 immediate early genes of herpes simplex virus (HSV) can critically determine the course of viral lytic or latent infections. Their promoters contain so-called TAATGARAT motifs that are activated via a multiprotein complex which includes cellular proteins Oct-1 and HCF and the viral activator VP16 (=Vmw65, αTIF). Relative to the ICP4 promoter TAATGAGAT sequence, the ICP0 promoter motif has a 5′ extension that includes a full octamer sequence (ATGCTAATGATAT). It seemed possible that this overlapping octamer site might render the ICP0 promoter element more active by allowing tighter binding of the Oct-1/VP16 complex or more vulnerable to repression by other Oct proteins. Our experiments favor the former possibility. On the one hand, the extended ICP0 site shows stronger binding of the Oct-1/VP16 complex compared to the ICP4 site. Moreover, transcription of a reporter gene with multiple ICPO sites is strongly activated by VP16 in transfected cells. On the other hand, the ICP0 site is largely refractory toward repression by a different Oct factor (N-Oct2 = Brn1) which competes with Oct-1/VP16 for the site. In marked contrast, multiple copies of the conventional TAATGAGAT motif of ICP4 are poorly activated by VP16, and transcription from this site can be completely repressed by N-Oct2. However, inclusion of the neighboring CGGAAR motifs from the ICP4 promoter, which bind factors GABPα and β, results in a strong synergistic activation. This activity, like that of the complete ICP4 promoter, becomes refractory to repression by competing N-Oct2. Thus the standard TAATGARAT motif of ICP4 is by itself less active and more vulnerable to repression than the extended ICP0 motif, and its activation depends upon synergism with neighboring DNA sites and their cognate factors. This difference between the two types of TAATGARAT motifs may allow for a more complex transcriptional regulation by factor combinations.