Trithorax- and Polycomb-group response elements within an Ultrabithorax transcription maintenance unit consist of closely situated but separable sequences

In Drosophila, two classes of genes, the trithorax group and the Polycomb g roup, are required in concert to maintain gene expression by regulating chr omatin structure. We have identified Trithorax protein (TRX) binding elemen ts within the bithorax complex and have found that within the bxd/pbx regul atory region these elements are functionally relevant for normal expression patterns in embryos and confer TRX binding in vivo. TRX was localized to t hree closely situated sites within a 3-kb chromatin maintenance unit with a modular structure. Results of an in vivo analysis showed that these DNA fr agments (each similar to 400 bp) contain both TRX- and Polycomb-group respo nse elements (TREs and PREs) and that in the context of the endogenous Ultr abithorax gene, all of these elements are essential for proper maintenance of expression in embryos. Dissection of one of these maintenance modules sh owed that TRX- and Polycomb-group responsiveness is conferred by neighborin g but separable DNA sequences, suggesting that independent protein complexe s are formed at their respective response elements. Furthermore, we have fo und that the activity of this TRE requires a sequence (similar to 90 bp) wh ich maps to within several tens of base pairs from the closest neighboring PRE and that the PRE activity in one of the elements may require a binding site for PHO, the protein product of the Polycomb-group gene pleiohomeotic. Our results show that long-range maintenance of Ultrabithorax expression r equires a complex element composed of cooperating modules, each capable of interacting with both positive and negative chromatin regulators.