MECHANISTIC ANALYSIS OF RNA-POLYMERASE-III REGULATION BY THE RETINOBLASTOMA PROTEIN

The tumour suppressor protein RB restricts cellular growth, This may i nvolve inhibiting the synthesis of tRNA and 5S rRNA by RNA polymerase (pol) III, We have shown previously that RE can repress pol III transc ription when overexpressed either in vitro or in vivo. We also demonst rated that pol III activity is elevated substantially in primary fibro blasts from RB-deficient mice. Here we address the molecular mechanism of this regulation, RE is shown to repress all types of pol III promo ter, It can do this even if added after transcription complex assembly , Functional assays demonstrate that RE targets specifically the gener al pol III factor TFIIIB, A physical interaction between TFIIIB and RE is indicated by fractionation, pull-down and immunoprecipitation data , We show that TFIIIB activity is elevated in primary fibroblasts from RB-deficient mice, TFIIIB is a multisubunit complex that includes the TATA-binding protein (TBP) and a TFIIB-related factor called BRF, We show that RE itself contains regions of homology to both TBP and BRF a nd propose a model in which RE disrupts TFIIIB by mimicking these two components.