The trithorax gene is required throughout development to maintain expression of homeotic genes of the bithorax and Antennapedia complexes. We determined complete structures of maternal and zygotic alternatively spliced trithorax transcripts, and found that two RNA isoforms are expressed in a surprising manner in the early embryo. At syncytial blastoderm their expression is confined to the ventral region fated to become mesoderm. An additional broad domain of trithorax expression arises during cellularization and is quickly resolved into four pair-rule-like stripes in the posterior half of the embryo. This early expression pattern suggested the possibility that trithorax is involved in the very early regulation of homeotic genes expressed only in the posterior region of the embryo. Indeed, transcription of bithorax complex genes in the mesoderm and ectoderm is altered in strong trithorax mutants during germ band elongation, while the anteriorly expressed Antennapedia complex genes are affected only at late stages of embryonic development. In addition, in another mutant allele (trxE3), expression of bithorax complex genes is normal, while expression of Antennapedia complex genes is reduced. These results suggest that proper expression of genes in the two homeotic complexes is maintained by products of different trithorax RNAs at different times of embryogenesis.

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