THE 3' REGULATORY REGION OF THE ABDOMINAL-B GENE - GENETIC-ANALYSIS SUPPORTS A MODEL OF REITERATED AND INTERCHANGEABLE REGULATORY ELEMENTS

The Abdominal-B (Abd-B) gene is one of three genes in the bithorax com plex, a cluster of homeotic genes in Drosophila. During embryogenesis Abd-B is expressed in a complex pattern, producing four different tran script classes, each of which exhibits a unique spatial pattern of exp ression. 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-B(Corset), w hich is caused by a deletion that leaves 15 kb of the 3' regulatory se quences immediately adjacent to the gene, but removes 45 kb of the mor e distant 3' regulatory elements. This mutation produces an unexpected homeotic segmental transformation of the fourth through seventh abdom inal segments, and has been analyzed by genetic and molecular techniqu es. In situ hybridization to Abd-B(Corset) embryos shows a uniform and moderate level of the Abd-B class A transcript in the posterior abdom en, rather than the normal graded pattern of expression. Our analysis of the Abd-B(Corset) mutation has prompted a model of the 3' regulator y region of Abd-B based on reiterated cell type-specific elements cont rolled by adjacent position-sensitive activating elements. The gradien t of Abd-B expression normally observed in the posterior abdomen appea rs to be achieved by varying the number of reiterated elements that ar e active in each segment.