HOMODIMERS AND HETERODIMERS OF THE RETINOID-X RECEPTOR (RXR) ACTIVATETRANSCRIPTION IN YEAST

The polymorphic nature of sequences which act as retinoic acid respons e elements (RAREs and RXREs) in transactivation assays in mammalian ce lls, suggests that elements consisting of a direct repetition of a hal f site motif, separated by 1 to 5 base pairs (DR1 to DR5), are targets for retinoic acid (RA) signalling. In a previous report we showed tha t in yeast cells, heterodimers of the retinoic acid receptors RARalpha and RXRalpha were required for efficient transcription of a reporter gene containing a DR5 element [Heery et al., (1993); Proc. Natl. Acad. Sci. USA, 90: 4281 - 4285]. Here we report that DR1 to DR5 elements c ontaining a direct repeat of the 5'-AGGTCA-3' motif, and an inverted r epeat of the same sequence with no spacer (IRO), behave as RAREs in ye ast cells coexpressing RARalpha and RXRalpha, albeit with different ef ficacies. Heterodimer activity was strongest on a DR5 reporter gene, a nd the strength of activation of the reporter series (DR5 > DR1 > DR3 > DR2 = IRO = DR4) correlated with the ability of the heterodimer to b ind to the corresponding sequences in vitro. Significantly, a reporter containing a DR1 element was selectively and efficiently activated in yeast cells expressing only RXRalpha. This activity was dependent on the induction by 9-cis retinoic acid of an activation function (AF-2) located in the RXRalpha ligand binding domain. In addition, a strong s ynergistic activity of RXRalpha was observed on a reporter containing the putative RXR element (RXRE) from the rat CRBPII gene promoter. Thu s, RXRalpha can function independently as a transcription factor, in t he absence of RARs or other heteromeric partners. Similarly, homodimer s of RARalpha selectively stimulated the transcription of a DR5 report er in a ligand-dependent manner, but less efficiently than RARalpha/RX Ralpha heterodimers.