Abstract
The Abdominal-B (Abd-B) gene is one of three genes in the bithorax complex, a cluster of homeotic genes in Drosophila. During embryogenesis Abd-B is expressed in a complex pattern, producing four different transcript classes, each of which exhibits a unique spatial pattern of expression. Proper regulation of the class A transcripts is required for appropriate development of the fifth through eighth abdominal segments and is mediated, in part, by a 60-kb regulatory region located 3' of the gene. We have isolated a new mutation, designated Abd-BCorset, which is caused by a deletion that leaves 15 kb of the 3' regulatory sequences immediately adjacent to the gene, but removes 45 kb of the more distant 3' regulatory elements. This mutation produces an unexpected homeotic segmental transformation of the fourth through seventh abdominal segments, and has been analyzed by genetic and molecular techniques. In situ hybridization to Abd-BCorset embryos shows a uniform and moderate level of the Abd-B class A transcript in the posterior abdomen, rather than the normal graded pattern of expression. Our analysis of the Abd-BCorset mutation has prompted a model of the 3' regulatory region of Abd-B based on reiterated cell type-specific elements controlled by adjacent position-sensitive activating elements. The gradient of Abd-B expression normally observed in the posterior abdomen appears to be achieved by varying the number of reiterated elements that are active in each segment.