What triggers zygotic gene expression in very early Drosophilaembryos?
Widespread zygotic expression begins at the blastoderm stage, but little is known about the regulation of the small numbers of genes transcribed in the pre-cellular blastoderm while the rest of the genome is still silent. Thomas Cline and colleagues (p. ) noticed that the sex determination genes - which have to be expressed early so that X-dosage compensation can kick in before widespread zygotic transcription begins - all possess multiple copies of the sequence CAGGTAG. Using a computational search, they found that this sequence, or similar degenerate sequences, are overrepresented upstream of most pre-blastoderm-expressed zygotic genes; they call these sequences the TAGteam. Remarkably, eliminating TAGteam sites causes the late transcription of genes carrying these sequences,and duplication of the minimal TAGteam sequence speeds up transcription. The authors suggest that the transcriptional regulation of these very early genes involves activators that bind the TAGteam motif, and they discuss approaches to identify these (probably maternally derived) regulatory proteins.
