Repeated turn-off and turn-on of fibroin gene transcription during silk gland development of Bombyx mori

Abstract

To measure the rates of fibroin mRNA synthesis in the posterior silk gland of Bombyx mori through the fourth feeding, fourth molting, and early fifth feeding stages, a saturation hybridization of 3H-RNA pulse labeled for 10 or 30 min to filter-bound fibroin DNA has been carried out. A high rate, 13 molecules/gene/min, of mRNA synthesis at 4 hr before the apolysis stage decreased precipitously to 0.44 and less than 0.084 molecule/gene/min at 4 and 7 hr after the apolysis, respectively. Such low rates, in the range of less than 0.006 to 0.084 molecule/gene/min, are observed for about 30 hr, from 7 hr after the apolysis to 5 hr after the fourth ecdysis, and are followed by a rapid elevation to 2.9 and 14 molecules/gene/min at 10 and 16 hr after the ecdysis, respectively. The rates of fibroin mRNA synthesis, if any, in the middle silk gland (fibroin nonproducer cells) are always less than 0.008 molecule/gene/min. These rates of fibroin mRNA synthesis are totally different from those of synthesis of ribosomal RNA and heterogeneous nuclear RNA of high molecular weight. From these results, we propose the possibility that fibroin gene expression is regulated at the transcriptional level through the repeated turn-off and turn-on of the gene during silk gland development.