REGULATION OF SILK GLAND PROTEINS IN THE SILKWORM, BOMBYX MORI (SERICIN, ECDYSONE)

Sericin is a protein which plays a structural role in formation of the cocoon of the silkworm, Bombyx mori. It, along with fibroin, account for more than 90% of the protein in the cocoon. Sericin is specific to the silk gland and when isolated from the silk gland at any time during late larval feeding periods consists of 9 peptides ranging in size from M(,r) 35,000 to M(,r) 230,000. The proteins are either utilized or degraded during the larval molting stages and synthesized during each feeding period. Sericin peptides of M(,r) 75000, M(,r) 135000 and M(,r) 230000 were shown to be closely related both immunologically and by peptide mapping. Partial DNA sequence information indicates that sericin contains tandem arrays of a 38 amino acid unit. The sericin peptides are not synthesized from separate genes as a single sericin RNA species is present in the silk gland at a time when all of the peptides are present. Instead the peptides contain domains consisting of stable regions which are separated by protease sensitive links. Genetic studies indicate that sericin shows size polymorphism derived from a single genetic locus. This size polymorphism is probably due to unequal crossing over events which leads to variability in the number of repetitive arrays.; cDNA clones to sericin were isolated and used as probes to measure sericin RNA abundance during silk gland development in the final two larval instars. Sericin RNA was found exclusively in the middle silk gland. The two sericin RNAs appear to be transcribed from a single gene and probably result from either alternative processing or site of polyadenylation. The steroid hormone ecdysone affects the abundance and size of these transcripts. Injection of 20-hydroxyecdysone results in a dramatic reduction in the accumulated level of both sericin and fibroin RNA. We hypothesize that fibroin and sericin RNA levels are regulated by circulating hormones, one of which is ecdysone. The two genes have different sensitivities to ecdysone such that low levels of the hormone affect sericin RNA transcripts but not fibroin transcripts. (Abstract shortened with permission of author.)...