EXPRESSION OF A CYTOPLASMIC ACTIN GENE IN RELATION TO THE SILK PRODUCTION CYCLE IN THE SILK GLANDS OF BOMBYX-MORI

The accumulation of transcripts of a cytoplasmic actin gene, the actin A 3 gene, has been studied in relation to the silk production cycle in the two secretory regions of the silk glands of Bombyx mori This accumulation was roughly similar in the two regions. The cellular amount of actin A3 mRNA decreased during the first period of the fourth moult, increased on and after the second period of this moult to reach a maximum at Day 5 of the fifth instar, then finally decreased. Actin biosynthesis almost completely ceased in the first period of the fourth moult and recovered in the second period of this moult. The accumulation of actin A3 mRNA parallels the development, in the silk gland cells, of the apical network of microfilaments which are implicated in silk secretion and which are disrupted at the beginning of the moult period and reorganized during the intermoult period. The accumulation of transcripts from actin A3 gene was compared to that of transcripts from silk protein genes and they were found to be asynchronous during the secretory cycle, indicating that mechanisms regulating expression of these two types of genes are different in the silk glands.