Fine organization of Bombyx mori fibroin heavy chain gene

The complete sequence of the Bombyx mori fibroin gene has been determined b y means of combining a shotgun sequencing strategy with physical map-based sequencing procedures. It consists of two exons (67 and 15 750 bp, respecti vely) and one intron (971 bp). The fibroin coding sequence presents a spect acular organization, with a highly repetitive and G-rich (similar to 45%) c ore flanked by non-repetitive 5' and 3' ends. This repetitive core is compo sed of alternate arrays of 12 repetitive and 11 amorphous domains. The sequ ences of the amorphous domains are evolutionarily conserved and the repetit ive domains differ from each other in length by a variety of tandem repeats of subdomains of similar to 208 bp which are reminiscent of the repetitive nucleosome organization. A typical composition of a subdomain is a cluster of repetitive units, Ua, followed by a cluster of units, Ub, (with a Ua:Ub ratio of 2:1) flanked by conserved boundary elements at the 3' end. Moreov er some repeats are also perfectly conserved at the peptide level indicatin g that the evolutionary pressure is not identical along the sequence. A ten tative model for the constitution and evolution of this unusual gene is dis cussed.