HIGHLY REPETITIVE STRUCTURE AND ITS ORGANIZATION OF THE SILK FIBROIN GENE

We have sequenced a number of cDNAs representing the Bombyx mori silk fibroin heavy chain transcript. These reveal that the central region o f the fibroin gene is composed of alternate arrays of the crystalline element a and the noncrystalline element b. The core region is partiti oned by a homogeneous nonrepetitive amorphous domain of around 100 bp in length. The element a is characterized by repeats of a highly conse rved 18-bp sequence coding for perfect repeats of the unit peptide Gly -Ala-Gly-Ala-Gly-Ser. The element b is composed of repeats of a less-c onserved 30-bp sequence which codes for a peptide similar to that in e lement a except in that(l) Ser is replaced by Tyr and (2) there are ir regular substitutions of Ala to Val or Tyr. Therefore, the structure o f the fibroin gene core consists of three-step higher-order periodicit ies. Heterogeneities in numbers of repeats are observed in each step o f periodicity. Boundary sequence appeared in each periodicity to be qu ite homogeneous. Sequence analysis indicates that the unit sequences o f elements a and b have homology to those of recombination hotspots re ported in other genes and a recombination event may frequently occur b etween the misaligned sister chromatids, resulting in heterogeneities in repeat numbers and duplication or deletion of repetitive sequences. The repetitive superstructure of the fibroin gene may have been a res ult of continuous unequal crossovers in a primordial gene during evolu tion. A couple of important features of the fibroin protein were prove d by the present nucleotide sequencing. The amino acid representation of the amorphous domain is vastly different from that of the repetitiv e regions. The carboxy-terminal nonrepetitive region has three Cys and nine (Arg + Lys) residues that may be responsible for complex formati on with the fibroin light-chain molecule. The present DNA analysis als o clearly demonstrates that the tRNA population in the posterior silk gland strictly complements the frequency of codons in the fibroin mRNA , which may help to achieve a highly efficient translation of fibroin mRNA.