REGULATION OF THE INFRAABDOMINAL REGIONS OF THE BITHORAX COMPLEX OF DROSOPHILA BY GAP GENES

The expression of the abdominal-A and Abdominal-B genes of the bithora x complex of Drosophila is controlled by cis-regulatory infraabdominal regions. The activation of these regions along the anteroposterior ax is of the embryo determines where abdominal-A and Abdominal-B are tran scribed. There is spatially restricted transcription of the infraabdom inal regions (infraabdominal transcripts) that may reflect this specif ic activation, We show that the gap genes hunchback, Kruppel, tailless and knirps control abdominal-A and Abdominal-B expression early in de velopment. The restriction of abdominal-ii and Abdominal-B transcripti on is preceded by (and requires) the spatially localized activation of regulatory regions, which can be detected by the distribution of infr aabdominal transcripts. The activation of these regions (except the in fraabdominal-8 one) could require no specific gap gene. Instead, a gen eral mechanism of activation, combined with repression by gap genes in the anteroposterior axis, seems to be responsible for delimiting infr aabdominal active domains. The gradients of the hunchback and Kruppel products seem to be key elements in this restricted activation.