Structures and organization of major plasma protein genes of the silkworm Bombyx mori

Abstract In the silkworm, Bombyx mori , a group of structurally related proteins, termed 30K proteins, accumulate in the hemolymph of the last instar larvae. We have isolated and characterized three genes, each of which encodes a distinct 30K protein component. Each 30K protein gene is composed of a short first exon and a protein-coding second exon interspersed by a single intron. The transcription initiation site of the 30K protein mRNA was identified at the nucleotide level. A typical TATA box exists some 30 base-pairs upstream from the transcription initiation site. The 5′-flanking region of each gene also contains octamer-like sequences. Restriction mapping analyses revealed that the cloned 46 × 10 3 base-pair region of the chromosomal DNA bears three 30K protein genes. Several copies of highly reiterated retrotransposon-like sequences are present around the 30K protein genes. S 1 nuclease protection analysis provided evidence that the biosynthesis of 30K protein is regulated in a stage-specific manner at the transcriptional level in the fat body.