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Retinoic acid receptor ␥ (RAR␥) is phosphorylated in COS-1 cells at two conserved serine residues located in the N-terminal region (serines 77 and 79 in RAR␥1 and serines 66 and 68 in RAR␥2) that contains the activation function AF-1. These serines are phosphorylated in vitro by cdk7, a cyclin-dependent kinase associated to cyclin H and MAT1 in the CAK complex (cdk7⅐cyclin H⅐MAT1), that is found either free or as a component of the transcription/DNA repair factor TFIIH. RAR␥ is more efficiently phosphorylated by TFIIH than by CAK and interacts not only with cdk7 but also with several additional subunits of TFIIH. RAR␥ phosphorylation and interaction with TFIIH occur in a ligand-independent manner. Our data demonstrate also that phosphorylation of the AF-1 function modulates RAR␥ transcriptional activity in a response gene-dependent manner.The pleiotropic effects of retinoids are transduced by two nuclear receptor families, the retinoic acid receptors (RARs) 1 and the retinoid X receptors (RXRs), that are ligand-dependent transregulators belonging to the nuclear receptor superfamily (1-4). RARs are activated by all-trans and 9-cis retinoic acid, whereas RXRs are activated by 9-cis retinoic acid only. There are three RAR (␣, ␤, and ␥) and three RXR (␣, ␤, and ␥) isotypes, and for each isotype there are at least two main isoforms that differ in their N-terminal region (1, 5, 6).As do other members of the nuclear receptor superfamily, RARs and RXRs exhibit a conserved modular structure with six variably conserved regions (A to F) (Fig. 1) (1, 5). The Nterminal A/B region of RARs contains a ligand-independent transcriptional activation function, AF-1 (7, 8). Although the B regions of the three RAR isotypes are moderately conserved, their A regions are unrelated and differ for each isoform of a given RAR isotype (5). The highly conserved C region encompasses the central DNA binding domain. The function of region F, if any, is unknown. Region E is more complex, as it contains the ligand binding domain, a dimerization interface, and the ligand-dependent transcriptional activation/repression domain AF-2 (1, 9). The activity of AF-2 is entirely dependent on the integrity of a conserved sequence referred to as the AF-2 AD core, located in ␣-helix 12 at the C-terminal end of the ligand binding domain. Ligand binding induces a major conformational change that includes helix 12 and creates a new surface for coactivator binding while corepressors are released, thus resulting in a transcriptional-competent nuclear receptor relayed to the transcriptional machinery and the chromatin template (1, 10 -12). The AF-2 and AF-1 activities synergize with each other in a response element-and promoter context-dependent manner (1,8,13).RARs and RXRs are phosphoproteins (14 -16), and their phosphorylation involves several kinases. RAR␣ can be phosphorylated in its AF-1-containing B region by the cyclin-dependent kinase cdk7 (14), which together with MAT1 and cyclin H forms the CAK complex that is found either free or as a component of the g...