Trithorax and dCBP acting in a complex to maintain expression of a homeotic gene

Trithorax (Trx) is a member of the trithorax group (trxG) of epigenetic reg ulators, which is required to maintain active states of Hox gene expression during development. We have purified from Drosophila embryos a trithorax a cetylation complex (TAC1) that contains Trx, dCBP, and Sbf1. Like CBP, TAC1 acetylates core histories in nucleosomes, suggesting that this activity ma y be important for epigenetic maintenance of gene activity. dCBP and Sbf1 a ssociate with specific sites on salivary gland polytene chromosomes, coloca lizing with many Trx binding sites. One of these is the site of the Hox gen e Ultrabithorax (Ubx). Mutations in either trx or the gene encoding dCBP re duce expression of the endogenous Ubx gene as well as of transgenes driven by the bxd regulatory region of Ubx. Thus Trx, dCBP, and Sbf1 are closely l inked, physically and functionally, in the maintenance of Hox gene expressi on.