Repression of basal transcription by HMG2 is counteracted by TFIIH-associated factors in an ATP-dependent process

A basal repressor of class II gene transcription was identified, purified, and found to be identical to nonhistone chromosomal protein HMG2. HMG2 was shown to inhibit basal transcription under conditions in which transcription templates form soluble complexes with HMG2. Order-of-addition experiments clearly revealed that HMG2 acted after assembly of a TBP-TFIIA-promoter complex and before formation of the fourth phosphodiester bond by RNA polymerase II. Subsequently, an activity that efficiently counteracted repression of transcription by HMG2 in both TBP- and TFIID-containing transcription systems was isolated. Several lines of evidence suggested that antirepression was mediated by a TFIIH-associated factor. The antirepressor first coeluted with TFIIH, was depleted from this fraction by antibodies directed against the TFIIH subunit p62, was dependent on either ATP or dATP, and then was inhibited by the ATP analogs AMP-PNP and ATP gamma S. Relief of HMG2-mediated repression as well as basal promoter function of TFIIH may involve a helicase that coelutes with TFIIH and displays similar nucleotide specificities. Taken together, these data suggest novel consequences of chromatin-associated HMG proteins and they provide direct evidence for a role of TFIIH-associated enzymes in ATP-dependent antirepression of nonhistone chromosomal proteins.