The expression of the abdominal-A and Abdominal-B genes of the bithorax complex of Drosophila is controlled by cis-regulatory infraabdominal regions. The activation of these regions along the anteroposterior axis of the embryo determines where abdominal-A and Abdominal-B are transcribed. There is spatially restricted transcription of the infraabdominal regions (infraabdominal transcripts) that may reflect this specific activation. We show that the gap genes hunchback, Kruppel, tailless and knirps control abdominal-A and Abdominal-B expression early in development. The restriction of abdominal-A and Abdominal-B transcription is preceded by (and requires) the spatially localized activation of regulatory regions, which can be detected by the distribution of infraabdominal transcripts. The activation of these regions (except the infraabdominal-8 one) could require no specific gap gene. Instead, a general mechanism of activation, combined with repression by gap genes in the anteroposterior axis, seems to be responsible for delimiting infraabdominal active domains. The gradients of the hunchback and Kruppel products seem to be key elements in this restricted activation.

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