THE DROSOPHILA TRITHORAX GENE ENCODES A CHROMOSOMAL PROTEIN AND DIRECTLY REGULATES THE REGION-SPECIFIC HOMEOTIC GENE FORK HEAD

The activity of the Drosophila gene trithorax is required to maintain the proper spatial pattern of expression of multiple homeotic genes of the Bithorax and Antennapedia complexes. trithorax encodes two large protein isoforms of >400 kD. We have detected its products at 16 discr ete sites on larval salivary gland polytene chromosomes, 12 of which c olocalize with binding sites of several Polycomb group proteins. The i ntensity of trithorax protein binding is strongly decreased in larvae carrying mutations in another trithorax group gene ash-1, and in the P olycomb group gene pco/E(z). A strong trithorax binding site was found at the cytological location of the fork head gene, a region-specific homeotic gene not located within a homeotic complex. Further analysis showed that trithorax protein binds at ectopic sites carrying fork hea d sequences in transformed lines. Trithorax binding occurs within an 8 .4-kb regulatory region that directs fork heed expression in several e mbryonic tissues including salivary glands. Consistently, expression o f endogenous fork head RNA is greatly reduced in trithorax mutant embr yos and in larval tissues. These results show that trithorax maintains expression of target genes by interaction with their regulatory regio ns and that this interaction depends on the presence of at least some of the other trithorax and Polycomb group proteins.