Structures and expression of mRNAs coding for major plasma proteins of Bombyx mori.

A group of structurally related proteins, referred to as '30K proteins', accumulate in a stage-dependent fashion in the larval hemolymph of the silkworm, Bombyx mori. To investigate the regulatory mechanisms of the 30K protein biosynthesis in the fat body, mRNA sequences coding for these plasma proteins were cloned and their structures and expression during the larval development were studied. Five plasmid clones, each bearing a distinct mRNA sequence coding for the 30K protein component, were isolated from a cDNA library constructed from the fat body poly(A)RNA of the mid-fifth instar larvae. These clones were remarkably similar to each other with respect to both the nucleotide and the amino-acid sequences. Each clone carried an open reading frame for amino-acid residues 251 to 264, with a typical signal sequence for transmembrane secretion at the deduced amino-terminal domain. Northern blot hybridization provided evidence that the developmental change in the amount of 30K protein mRNAs in the fat body reflects well that of the hemolymph concentration of 30K proteins, indicating that the biosynthesis of 30K proteins is developmentally regulated at a transcriptional or post-transcriptional level in the fat body.