Different behavior of chromatin domains encompassing fibroin heavy-chain gene in active, temporarily inactive, and permanently inactive transcriptional states in silk gland nuclei.

Chromatin structures of an approximately 46-kilobase pair region encompassing fibroin heavy chain gene of the silkworm, Bombyx mori, were compared in active (in the posterior silk gland nuclei of the fifth instar larvae), temporarily inactive (in the posterior silk gland nuclei of the fourth molting stage larvae), and permanently inactive (in the middle silk gland nuclei of the fifth instar larvae) transcriptional states. Chromatin structure of the second exon, the major body of the protein-encoding region, was highly sensitive to both DNase I and methidiumpropyl EDTA-Fe(II) in the active state but highly resistant to those agents in both temporarily and permanently inactive states, except that the fibroin "amorphous" region-encoding subregions remained relatively sensitive to DNase I. Distributions of DNase I hypersensitive sites in the 5' upstream region were generally similar between active and temporarily inactive states, but those of the permanently inactive state were markedly different. In the promoter-enhancer region, phosphodiester bonds between nucleotide positions -41 and -42 and also -42 and -43 were remarkably hypersensitive to DNase I in the active chromatin, but the same positions in the other two states were not.