Mori K, Tanaka K, Kikuchi Y, Waga M, Waga S, Mizuno S
Department of Applied Biological Chemistry, Faculty of Agriculture, Tohoku University, Sendai, Japan.
J Mol Biol. 1995 Aug 11;251(2):217-28. doi: 10.1006/jmbi.1995.0429.
The allelic Nd-s and Nd-sD 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-sD 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 and Nd-sD 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 for one base change in the coding region. The N-terminal half of the L-chain 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-terminal half of the Nd-sD mutant L-chain is encoded by two exons, IV' and V', which are brought into proximity with the exon III by recombination between sequences in the third intron and in the far downstream region with concomitant loss of a region containing exons IV to VII. Sequences corresponding to exons IV' and V' are present about 10 kb downstream of the L-chain gene in the J-139 genome. Their homologous sequences 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-sD strains without disulfide-bonding to the fibroin heavy (H-) chain, as revealed by Western blotting with the antibody specific to the C-terminal half of the mutant L-chain.
家蚕等位基因Nd-s和Nd-sD突变被定位到与丝素轻链（L链）基因（Fib-L）相同的位点。携带纯合Nd-s或Nd-sD突变的家蚕分泌的丝素不到正常水平的0.3%，并产生薄茧（裸蛹）。在本研究中，将Nd-s和Nd-sD L链的cDNA序列与家蚕J-139品系（丝素正常水平生产者）L链的cDNA和基因组序列进行了比较。除编码区有一个碱基变化外，两个突变cDNA序列几乎相同。由外显子I至III编码的L链N端一半在突变体和J-139之间是相同的，但分子的其余部分完全不同。Nd-sD突变L链的C端一半由两个外显子IV'和V'编码，它们通过第三个内含子和远下游区域的序列之间的重组与外显子III靠近，同时包含外显子IV至VII的区域缺失。与外显子IV'和V'对应的序列存在于J-139基因组中L链基因下游约10 kb处。在DNA和蛋白质数据库中未发现它们的同源序列。如用针对突变L链C端一半的抗体进行蛋白质免疫印迹所显示，约27 kDa的嵌合L链分子存在于Nd-s和Nd-sD品系的后部丝腺中，且未与丝素重链（H链）形成二硫键。
