CLONING AND CHARACTERIZATION OF A TFIIIC2 SUBUNIT (TFIIIC-BETA) WHOSE PRESENCE CORRELATES WITH ACTIVATION OF RNA-POLYMERASE III-MEDIATED TRANSCRIPTION BY ADENOVIRUS E1A EXPRESSION AND SERUM FACTORS

TFIIIC2 is a general factor essential for transcription of 5S RNA, tRNA, and VA RNA genes by mammalian RNA polymerase III and consists of two forms designated TFIIIC2a and TFIIIC2b. TFIIIC2a and TFIIIC2b share common subunits of 220, 102, 90, and 63 kD but differ with respect to transcription activity and the presence of a presumptive 110-kD subunit in the active form (TFIIIC2a). Because both forms can bind the promoter directly, a selective role for the 110-kD subunit in the regulation of RNA polymerase III activity has been suggested. To investigate this possibility, we have cloned and expressed a cDNA encoding the 110-kD subunit (TFIIIC beta). Immunoprecipitation studies with anti-TFIIIC beta antibodies have confirmed that TFIIIC beta is a bona fide subunit present only in TFIIIC2a, that TFIIIC2a and the general factor TFIIIC1 are associated in unfractionated extracts, and that previously undetected polypeptides (potential TFIIIC1 subunits) can be isolated in association with TFIIIC2a. Previous studies have shown that increases in RNA polymerase III activity during infection of cells by adenovirus (with concomitant E1A expression) or during cell growth at high serum concentration results from an increased activity in the TFIIIC fraction. Studies with antibodies to TFIIIC beta have shown that this is strongly correlated with a selective increase in the cellular concentration of the TFIIIC beta 110-kD subunit and a concomitant rise in the ratio of the active-to-inactive forms of TFIIIC2.