Fine organization of Bombyx mori fibroin heavy chain gene

TLDR

The study proposes a tentative model for the constitution and evolution of the Bombyx mori fibroin heavy chain gene. The gene was fully sequenced using a combination of shotgun sequencing and physical map‑based approaches. The gene comprises two exons (67 bp and 15,750 bp) separated by a 971‑bp intron, with a highly repetitive, G‑rich core flanked by non‑repetitive ends; the core contains alternating arrays of 12 repetitive and 11 amorphous domains, each subdomain composed of clusters of Ua and Ub units (ratio 2:1) with conserved boundaries, and some repeats are perfectly conserved at the peptide level, indicating variable evolutionary pressure.

Abstract

The complete sequence of the Bombyx mori fibroin gene has been determined by means of combining a shotgun sequencing strategy with physical map-based sequencing procedures. It consists of two exons (67 and 15 750 bp, respectively) and one intron (971 bp). The fibroin coding sequence presents a spectacular organization, with a highly repetitive and G-rich (~45%) core flanked by non-repetitive 5′ and 3′ ends. This repetitive core is composed of alternate arrays of 12 repetitive and 11 amorphous domains. The sequences of the amorphous domains are evolutionarily conserved and the repetitive domains differ from each other in length by a variety of tandem repeats of subdomains of ~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. Moreover some repeats are also perfectly conserved at the peptide level indicating that the evolutionary pressure is not identical along the sequence. A tentative model for the constitution and evolution of this unusual gene is discussed.