Silkmoth chorion antisense RNA. Structural characterization, developmental regulation and evolutionary conservation.

Choriogenic follicular cells of the silkmoth Bombyx mori contain significant quantities of antisense RNA transcribed from chorion genes. Antisense RNA derived from a chorion gene with a high content of cysteine, HcB.12, was characterized in detail. The antisense transcripts are initiated downstream from the 3' end of HcB.12 mRNA and extend over 75% of the length of the gene, comprising its entire second exon and part of its intervening sequence. The antisense RNA is devoid of any significant open reading frames and is not polyadenylated. These features, combined with the presence of specific sequence motifs within its transcribed and upstream region, suggest that antisense RNA may be transcribed by RNA polymerase III. Chorion antisense RNA is detectable only in choriogenic follicular cells and appears to be co-ordinately regulated with chorion mRNA. Its cytoplasmic accumulation during choriogenesis parallels that of the corresponding mRNA. Although chorion mRNA is at least five times more abundant than antisense RNA, the latter is present as a single-stranded entity in follicular cytoplasm but can form perfect duplexes with its mRNA complement upon annealing in vitro. The possible involvement of antisense RNA transcription in the pathway that controls the programmed expression of chorion genes at the level of transcription initiation or post-transcriptional processing is discussed.