[Gene expression in Bombyx mori silkgland cells during the last larval instar].

Poly (A)-containing RNA was isolated from the posterior silkgland of Bombyx mori just after the fourth molt, when the gland is not producing fibroin (stage V0), and at the middle of the fifth larval instar, when the fibroin is massively synthesized (stage V6). The hybridization kinetics of these mRNA with their complementary DNA (cDNA) were remarkably different. The mRNA from stage V0 consisted of two abundance classes, comprising 38 and 2 915 different species, respectively. At stage V6, fibroin mRNA (FmRNA) was separated from the rest of the poly (A)-containing RNA (F-mRNA) by centrifugation on sucrose gradients, and both preparations were analyzed separately. The kinetic complexity of FmRNA was very low as compared to its actual size. This result agrees with the existence of a short repetitive sequence accounting for 60 p. 100 of the molecule. Stage V6 F-mRNA was resolved into four classes containing 1, 20, 319 and about 2 600 species, respectively. We carried out cross-hybridization with each of the two cDNA classes from stage V0 and stage V6 F-mRNA. They demonstrated that almost all the sequences present at stage V0 were also present at stage V6, but that their abundance class distribution was modified. Our data show that the specialization of the silkgland cell for fibroin production is characterized by massive accumulation of FmRNA and another unknown species, and that stage variations of mRNA populations are related to relative quantitative variations rather than to qualitative ones.