Abstract
The insect steroid hormone, ecdysone, initiates larval ecdysis and metamorphosis in many insects by activating a cascade of genes—beginning with early genes, most of which encode transcriptional regulators. One of the early genes, Broad-Complex (BR-C) encodes a BTB zinc-finger protein that exists as a number of isoforms generated by alternative splicing. The isoforms are categorized into 4 classes (Z1 to 4) according to the sequence of their zinc-finger domain. A BR-C Z3-isoform is expressed in the Drosophila larval salivary gland and fat body. By contrast, in Bombyx, the BR-C homologue (BmBR-C) Z3-isoform has not yet been detected, even though a Z3-type zinc-finger coding sequence is found in the genome.
In this study, cDNAs encoding the Z2-isoform, carrying a partial (Z2/3s) or a full-length (Z2/3) sequence of an additional exon (exon 13), including an out of frame Z3-type zinc-finger coding sequence were cloned. BmBR-C is transcribed from two different promoters. One is the ecdysone-inducible distal promoter (Pdist), and the other is the ecdysone-noninducible proximal promoter (Pprox). When the broad category of Z2-isoforms was abundantly expressed from either promoter, the transcripts were predominantly composed of the Z2/3-isoform. In these mRNAs, polyadenylation signals of the “variant type”, located proximal to the coding sequence were used for the Z2-isoform transcripts. Additionally, one of the canonical signals, “AUUAAA”, or the variant type signals located at the 3’-most site was used as a polyadenylation signal for the Z2/3-isoform transcripts. The different signal usage may play a role in the polyadenylation rate of the Z2 and Z2/3-isoforms.
