2 SILENCING SUB-DOMAINS OF V-ERBA SYNERGIZE WITH EACH OTHER, BUT NOT WITH RXR

The thyroid hormone receptor (TR) and the retinoic acid receptor (RAR) induce gene expression in the presence of specific ligand and repress transcription in the absence of hormone. This repression is mediated by an active silencing mechanism rather then by interference with DNA binding activators. V-erbA, a variant form of TR which is unable to bi nd hormone, represents a constitutive repressor. Here we show, using f usion proteins with the GAL4 DNA binding domain, that the minimal sile ncing domain of v-erbA extends from amino acids 389 to 632 and that in ternal deletions within this domain retain at least some repression fu nction. Co-transfection experiments of different deletion mutants indi cate that the silencing domain is composed of at least two sub-domains which are non-functional when tested individually. When combined in a heterodimeric complex, they synergize such that silencing activity is regained. In contrast to the retinoic acid receptor the retinoid X re ceptor does not contain a silencing domain. In addition it is unable t o cooperate with the repression function of TR or v-erbA in a heterodi mer.