Cloning and molecular characterization of the trithorax locus of Drosophila melanogaster.

The trithorax (trx) locus of Drosophila melanogaster affects segment determination primarily in the thoracic region. Mutant flies show transformations of the third and, to a lesser extent, first thoracic segment toward the second thoracic segment; abdominal transformations also occur. Prior genetic evidence suggested that these effects are based on interactions between trx and genes of the bithorax complex and Antennapedia complex. Further, interactions between the maternal effect locus female sterile homeotic (fsh) and trx have been observed. To aid in a molecular analysis of trx function, we have cloned the locus by a P-element transposon tagging approach. Five insertion mutations have been mapped within a region of about 10 kilobases; one of these mutations reverted coincident with the loss of the insertion. Transcription mapping suggests that two RNAs of about 12 and 15 kilobases are the major transcripts of the trx locus and that the transcription unit comprises a region of about 25 kilobases. Transcripts from the trx locus are distributed uniformly in early embryos, but at 14-16 hr after fertilization the ventral nerve cord contains a higher concentration of trx RNA than other regions of the embryo.