PRODUCTION OF A CHIMERIC FIBROIN LIGHT-CHAIN POLYPEPTIDE IN A FIBROINSECRETION-DEFICIENT NAKED PUPA MUTANT OF THE SILKWORM BOMBYX-MORI

The allelic Nd-s and Nd-s(D) mutations of the silkworm Bombyx mori are mapped to the same locus as that of the fibroin light (L)-chain gene (Fib-L). The silkworm carrying the homozygous Nd-s or Nd-s(D) mutation secretes less than 0.3% of the normal level of fibroin and produces a thin cocoon (naked pupa). In this study, cDNA sequences of the Nd-s a nd Nd-s(D) L-chains were compared with the cDNA and genomic sequences of the L-chain of the B. mori J-139 strain, a normal-level producer of fibroin. The two mutant cDNA sequences are almost identical except fo r one base change in the coding region. The N-terminal half of the L-c hain encoded by exons I to III is identical between the mutants and J- 139, but the rest of the molecule is completely different. The C-termi nal half of the Nd-s(D) mutant L-chain is encoded by two exons, IV' an d V', which are brought into proximity with the exon III by recombinat ion between sequences in the third intron and in the far downstream re gion with concomitant loss of a region containing exons IV to VII. Seq uences corresponding to exons IV' and V' are present about 10 kb downs tream of the L-chain gene in the J-139 genome. Their homologous sequen ces have not been found in the DNA and protein databases. The chimeric L-chain molecule of about 27 kDa is present in posterior silk glands of Nd-s and Nd-s(D) strains without disulfide-bonding to the fibroin h eavy (H-) chain, as revealed by Western blotting with the antibody spe cific to the C-terminal half of the mutant L-chain. (C) 1995 Academic Press Limited