RNA-POLYMERASE-III TRANSCRIPTION REPRESSED BY RB THROUGH ITS INTERACTIONS WITH TFIIIB AND TFIIIC2

The retinoblastoma susceptibility gene product (Rb) generally represse s RNA polymerase Ill (Pol III)-directed transcription. This implies th at Rb interacts with essential transcription factors. Mutations in eit her the A or B subdomains in the Rb pocket interfere with Rb-mediated repression of Pol III-directed transcription, which indicates that bot h subdomains are directly involved in this activity. Addition of eithe r purified TFIIIB or purified TFIIIC2 partially relieves Rb-mediated r epression and restores activity to nuclear extracts that had been depl eted of essential factors by binding to Rb. Pull down and coimmunoprec ipitation experiments as web as functional assays indicate that Rb int eracts with both TFIIIB and TFIIIC2 and that the A subdomain is primar ily required for binding TFIIIB and the B subdomain for binding TFIIIC 2. While Rb interacts with both factors, the A subdomain is more impor tant than the B subdomain in directing Rb-mediated repression, and TFI IIB is the principal target of that activity.