Using fractionated HeLa cell nuclear extracts and both nuclease (DNase I) cleavage and chemical cleavage (methidiumpropyl-EDTA X Fe(II) protection methodologies, we demonstrated the presence of three proteins which interacted specifically, yet differentially, with the two VA genes of adenovirus type 2. One, previously identified as transcription initiation factor TFIIIC, bound to a site centered on the transcriptionally essential B-block concensus element of the VAI gene and, with a lower affinity, to the analogous site in the VAII gene. Another, identified as the cellular protein involved in adenovirus replication, nuclear factor I, bound to sites immediately downstream from the two VAI terminators (at approximately +160 and +200). The third, a previously unrecognized VA gene binding protein termed VBP, bound immediately upstream of the B-block element in the VAI gene but showed no binding to VAII. Possible roles for these proteins in VA gene transcription were investigated in in vitro assay systems reconstituted with partially purified transcription factors (RNA polymerase III, TFIIIB, and TFIIIC). Although TFIIIC activity was present predominantly in fractions containing B-block binding activity, there was not complete correspondence between functional and DNA binding activities. The nuclear factor I-like protein had no effect when added to a complete transcription reaction. The presence of VBP appeared to depress the intrinsic ratio of VAI-VAII synthesis, thereby simulating the relative transcription levels observed early in adenovirus infection of HeLa cells. These observations suggest a model, involving both intragenic binding factors (VBP and TFIIIC) and variable template concentrations, for the differential regulation of VA transcription during the course of adenovirus infection.
Nuclear factor I is specifically targeted to discrete subnuclear sites in adenovirus type 2-infected cells.
