THE ACIDIC TRANSCRIPTIONAL ACTIVATION DOMAINS OF HERPES-VIRUS VP16 AND YEAST HAP4 HAVE DIFFERENT COFACTOR REQUIREMENTS

The acidic transcriptional activation domain of the herpes virus activ ator VP16 requires an accessory protein complex for function, termed a n adaptor. Although the activation domain of the yeast activator HAP4 is also highly negatively charged, its function is independent of at l east one component of the adaptor complex, ADA2 In this study, we have used an in vitro inhibition assay to determine whether the activation domains of VP16 and HAP4 use a similar mechanism to potentiate transc ription. Both domains had potent activation ability, indicating a simi lar strength of action. However, the capacity of each domain to inhibi t activation of a heterologous test promoter (dA/dT) was sharply dissi milar. VP16 selectively inhibited activated transcription of dA/dT, wi thout affecting basal transcription, implying that VP16 and the activa tor protein of the dA/dT promoter share a mechanism for activation, In contrast, HAP4 was totally unable to inhibit activated transcription of the dA/dT template, In the second part of the study, a genetic sele ction was used to obtain mutations in putative cofactor genes for HAP4 . The spectrum of phenotypes caused by these mutations was strikingly different than mutations in the adaptor for the VP16 activation domain . These results strongly suggest that HAP4 and VP16 have distinct cofa ctor requirements, although they are both acidic activators.