Two 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 bind hormone, represents a constitutive repressor. Here we show, using fusion proteins with the GAL4 DNA binding domain, that the minimal silencing domain of v-erbA extends from amino acids 389 to 632 and that internal deletions within this domain retain at least some repression function. Co-transfection experiments of different deletion mutants indicate 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 receptor does not contain a silencing domain. In addition it is unable to cooperate with the repression function of TR or v-erbA in a heterodimer