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 discrete sites on larval salivary gland polytene chromosomes, 12 of which colocalize with binding sites of several Poly comb group proteins. The intensity of trithorax protein binding is strongly decreased in larvae carrying mutations in another trithorax group gene ash-1, and in the Polycomb 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 head sequences in transformed lines. Trithorax binding occurs within an 8.4-kb regulatory region that directs fork head expression in several embryonic tissues including salivary glands. Consistently, expression of endogenous fork head RNA is greatly reduced in trithorax mutant embryos and in larval tissues. These results show that trithorax maintains expression of target genes by interaction with their regulatory regions and that this interaction depends on the presence of at least some of the other trithorax and Polycomb group proteins.[Key Words: Drosophila-, trithorax-, chromosomal binding; regulation of expression; fork head]

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The gypsy (mdg4) mobile element of Drosophila contains two closely spaced regions which bind proteins from nuclear extracts. One of these is an imperfect palindrome having homology with the lac‐operator of Escherichia coli; the other contains a reiterated sequence (5′PyPuT/C TGCATAC/TPyPy) homologous to the octamer that is the core of many enhancers and upstream promoter elements. Transient expression of deletion mutants has shown that these DNA regions are negative and positive regulators of transcription. As was demonstrated earlier by other authors, mutations induced by the presence of gypsy in different loci are suppressed owing to either repression or activation of gypsy transcription in Drosophila strains carrying unlinked mutations in su(Hw) or su(f) genes. We have shown that binding to a negative regulator (silencer) is weakened in nuclear extracts isolated from fly stocks carrying su(f) mutations which activate gypsy transcription; therefore the su(f) gene seems to code for a protein capable of gypsy repression. Furthermore, binding to a positive regulator is weakened in nuclear extracts isolated from fly stocks carrying su(Hw) gene mutations which decrease the level of gypsy transcription; therefore, the su(Hw) gene most likely encodes a protein which activates gypsy transcription.

We addressed the possibility of the horizontal transfer of long interspersed element (LINE)-like mobile elements by studying the distribution of the Drosophila melanogaster LINE-like elementjockey in different Drosophila species. Outside the D. melanogaster groupjockey was detected only in the distantly related species Drosophila funebris. Cloning and sequencing of this element from D. funebris revealed the existence of the two open reading frames highly similar to those ofjockey from D. melanogaster. Elements from both species are transcriptionally active and contain in their promoter regions a conserved sequence important for its activity. The high degree of similarity between the D. melanogaster and the D. funebris jockey and the absence of jockey from other sibling species of the D. funebris group provide evidence for the horizontal transmission ofjockey into D. funebris.