STRUCTURE AND EXPRESSION OF THE BR-C LOCUS IN DROSOPHILA-MELANOGASTER(DIPTERA, DROSOPHILIDAE)

We review data on the structure and expression of the Broad Complex, a key gene in the response to ecdysterone (20E) in Drosophila cells. Th e 2B3-7 X chromosome region, containing an early (20E) inducible puff, was saturated with mutations in the genes dor, hfw (swi) and BR-C tha t cause disturbances in the normal ecdysterone-inducible puffing seque nce. By analysis of chromosome rearrangements localized on the genetic , cytological and physical maps, the BR-C locus has been mapped exactl y within the 2B3-5 puff. Homozygous mutations for BR-C result in the m ultiple distortions of metamorphosis and female fertility due to loss of sensitivity of the genes in this chromosomal region to ecdysteroids . Using interstrain larval ovary transplantation, the causes of female sterility were studied in females mutant for the BR-C. Transplantatio n of BR-C ovaries into females heterozygous for a dominant sterility m utation followed by genetic analysis of the progeny demonstrated that the transplanted ovaries were capable of normal function. The sterilit y of the BR-C mutant females is therefore associated with a somatic ex tra-ovarial defect rather than a germ line malfunction. According to c ytogenetic and molecular analysis, the BR-C occupies more than 115 kb and contains at least three regulatory elements. One of these elements (in position 146.5-161 kb) may respond to 20E and this results in puf f activation in the salivary gland cells. The second element (at 99-12 0 kb) is located upstream in the first exon and is necessary for trans cription activation, while the third element (to the left of 99 kb) is necessary for normal fertility. The strongest BR-C mutations result i n underdevelopment of almost all ecdysterone-inducible puffs. In anima ls homozygous for the weaker BR-C mutations (rbp group) which survive until organ formation in the pupa, the larval salivary glands do not h istolyse until at least 72 hour after pupariation (Restifo and White, 1992). Analysis of polytene chromosomes has shown that in the rbp(4) m utant all ecdysterone-inducible puffs are extremely reduced in 0, 3, a nd 10 h prepupae. Some of the puffs including the 2B3-5 are active unt il 24 h pupae.