Negative and positive regulators modulate the activity of a silkmoth chorion gene during choriogenesis

We have used in vitro linker substitution mutagenesis and P-element-mediated transformation in Drosophila to define cis-regulatory elements of the bidirectional promoter of the Bombyx mori A/B.L12 chorion genes in an in vivo situation. Within an essential 112 bp part of this promoter, mutations in two non-contiguous approximately 35 base-pair regions (A1 and A2) result in significant reduction of expression at late choriogenesis. The reduction occurs in both promoter orientations, indicating the existence of bidirectionally active positive elements. Mutations in A1, but not in A2, also lead to premature activation at early choriogenesis, suggesting the existence of a negative element essential for the finely tuned expression profile during choriogenesis. Normally, this element apparently prevents early expression; it also acts bidirectionally. The highly specific effects of these mutations suggest that several trans-acting regulators of chorion gene transcription have been conserved between silkmoths and fruitflies.