Developmental Regulation of Silk Gene Expression in Bombyx mori

Abstract

Our major concern has been understanding how cells are specialized and express a set of genes during development. Along this line, our target has been a dissection of silk gene regulation in the silk gland of Bombyx mori (Suzuki, 1977; Suzuki et al., 1987). The development of the silk gland originates as an invagination of the ectoderm in the labial segment of stage 19 embryos, and completes morphologically by stage 25 (Nunome, 1937). Transcription of the fibroin (heavy chain) gene (Suzuki and Brown, 1972; Suzuki et al., 1972; Ohshima and Suzuki, 1977; Tsujimoto and Suzuki, 1979a, b) begins at around stage 25 of the embryos (Ohta et al., 1988). After this first activation the fibroin gene is repeatedly switched on and off in the posterior silk gland cells during larval development (Suzuki and Suzuki, 1974; Suzuki and Giza, 1976; Maekawa and Suzuki, 1980). When fibroin gene transcription in the posterior silk glands of the fifth instar larvae was analyzed by nuclear run-on assays, it was found that the transcription is restricted to the anterior region at the beginning of the instar and spreads toward the posterior region as the stage proceeds (Obara and Suzuki, 1988). Transcription of the fibroin light chain (or P25) gene was found to occur in parallel with that of the fibroin gene during the fourth molting stage and the fifth larval instar (Couble et al., 1983; Kimura et al., 1985).