Structural analysis of sericin genes. Homologies with fibroin gene in the 5' flanking nucleotide sequences.

Two closely related mRNAs (11.0 and 9.6 kilobases long) for sericin, a cocoon protein other than fibroin, were isolated from the middle silk gland of Bombyx mori. They were shown to be specifically accumulated in this particular part of the gland. Nine genomic clones complementary to the mRNAs were isolated from the recombinant phage library of Bombyx DNA. They represent two structurally similar genomic segments, and contain the 5' end and its surrounding region of sericin gene. We have identified at least 5 exons in then, and the intervening coding junctions were determined at nucleotide level. The two genomic segments differ mainly in the 5' flanking as well as in the intervening sequences and may represent alleles of a sericin locus. DNA sequence analysis of an exon has revealed an internally repetitive unit of 114 base pairs. From an internally repetitive unit of 114 base pairs. From an open frame of the repetitive DNA sequence, a peptide consisting of 38 amino acids abundant in serine was deduced. We propose the peptide represents a repetitive unit of a sericin protein. The probable site of initiation of sericin gene transcription in vivo has been identified at the nucleotide level that also turned out to be the 5' end of mature sericin mRNA. There exist strong homologous regions in the sericin and fibroin genes at their corresponding 5' flanking sequences. A possible involvement of the homologous sequences around -111 and -186 for the concerted expression of the sericin and fibroin genes in the silk glands is discussed.