FUNCTIONAL RECONSTRUCTION OF TRANS REGULATION OF THE ULTRABITHORAX - PROMOTER BY THE PRODUCTS OF 2 ANTAGONISTIC GENES, TRITHORAX AND POLYCOMB

Maintenance of the ''on-of'' state of Drosophila homeotic genes in Ant ennapedia and bithorax complexes requires activities of the trithorax and Polycomb groups of genes. To identify cis-acting sequences for fun ctional reconstruction of regulation by both trithorax and Polycomb, w e examined the expression patterns of several Ubx-lacZ transgenes that carry upstream fragments corresponding to a region of approximately 5 0 kb. A 14.5-kb fragment from the postbithorax/bithoraxoid region of U ltrabithorax exhibited proper regulation by both trithorax and Polycom b in the embryonic central nervous system. Using a Drosophila haploid cell line for transient expression, we found that trithorax or Polycom b can function independently through this upstream fragment to activat e or repress the Ultrabithorax promoter, respectively. Studies of dele tion mutants of trithorax and Polycomb demonstrated that trithorax-dep endent activation requires the central zinc-binding domain, while Poly comb-dependent repression requires the intact chromodomain. In additio n, trithorax-dependent activity can be abrogated by increasing the amo unt of Polycomb, suggesting a competitive interaction between the prod ucts of trithorax and Polycomb. Deletion analysis of this fragment dem onstrated that a 440-bp fragment contains response elements for both t rithorax and Polycomb. Furthermore, we showed that the integrity of th e proximal promoter region is essential for trithorax-dependent activa tion, implicating a long-range interaction for promoter activation.