Structure and expression of a gene coding for egg-specific protein in the silkworm, Bombyx mori

Abstract

Egg-specific protein (ESP) is a yolk protein which is synthesized in developing ovarian follicles of the silkworm, Bombyx mori. We have isolated a gene coding for ESP from a genomic DNA library using cDNA as a probe and sequenced about 2.9 kb including 5′- and 3′-flanking regions. Three closely located sites were identified as transcription initiation sites by primer extension analysis. In the 5′-upstream region, there are several short sequences which are homologous to the regulatory elements of other genes: short sequences homologous to the ecdysteroid response elements of the heat shock protein genes and the glue protein genes, cis- acting elements of chorion genes, and a protein factor binding site of the fibroin gene. Southern hybridization analysis revealed the presence of a single copy of the ESP gene per a haploid genome. The amino acid sequence was highly homologous to the human gastric and rat lingual lipases. Northern hybridization analysis of RNA prepared from various tissues at different developmental stages showed that the ESP gene is expressed only in follicles undergoing vitellogenesis. Injection of 20-hydroxyecdysone into the isolated pupal abdomen stimulated the expression of the ESP gene with a lag period of 2 days.