Differential usage of two promoters of the Broad-Complex gene in the silkworm, Bombyx mori

The Broad-Complex gene encodes one of the key regulators of the ecdysone signal cascade. We previously isolated part of the genomic DNA and cDNAs of Broad-Complex in Bombyx mori (BmBR-C). Here, we report structures of the entire genomic DNA and 5' untranslated region (5'-UTR) of the cDNAs. BmBR-C was found to span about 158 kbp including 13 exons. In the 5'-UTR, additional alternatively spliced exons were identified. The 5' ends of the cDNAs were mapped to two different positions, the distal promoter (P(dist)) and proximal promoter (P(prox)), separated by 86 kbp. Expression from these promoters was controlled differentially. Semi-quantitative PCR using cDNAs from the carcass, silk glands and fat body revealed that expression from P(prox) was changed moderately and expression from P(dist) was weak and constant during the fourth ecdysis. At the onset of pupation, expression from P(prox) was suppressed in all tissues, but that from P(dist) was induced in the carcass and ASG. In the fat body, expression from both promoters increased in the prepupal stage. A combination of promoters differing in responsiveness to an ecdysone signal may serve to achieve a complex regulation of downstream genes in reply to a simple hormonal signal.