Post-transcriptional polyadenylation is probably an essential step in selection of Balbiani ring transcripts for a cytoplasmic role.

Abstract

Some aspects of poly(A) addition on to the RNA of the two major Balbiani rings 1 and 2 and of the fate of polyadenylated and nonpolyadenylated 75-S RNA transcripts in salivary gland cells of Chironomus tentans have been investigated. RNA derived from the Balbiani ring portion of chromosome IV, essentially representing a spectrum of nascent growing transcripts, lacks poly(A) segments, while about 16% of the finished 75-S RNA molecules released into the nuclear sap is associated with poly(A) stretches. More than 90% of the recently arrived cytoplasmic 75-S RNA molecules contain poly(A) chains that consist of about 103 AMP residues. Poly(A)-containing 75-S RNA rapidly disappears from the nucleus and no significant amount of nonpolyadenylated Balbiani ring RNA molecules can be chased into the cytoplasm. A large proportion of Balbiani ring transcripts remains without poly(A) segments in the nuclear sap even after prolonged incubation periods and probably never leaves the nucleus. It seems probable therefore that the molecular mechanism operating in the selection of 75-S RNA chains for cytoplasmic use includes post-transcriptional poly(A) addition as an obligatory event in the biogenesis of Balbiani ring RNA. Another defined mRNA fraction of non-Balbiani-ring origin, designated 35-S RNA, which emerges in the nuclear sap after 45 min of labelling with tritiated RNA precursors, appears to be polyadenylated up to 100% and becomes to a large extent converted to cytoplasmic 35-S RNA. A small but significant proportion, about 13%, of hnRNA produced by non-Balbiani-ring genes on the chromosomes I–III is polyadenylated. The corresponding figure for hnRNA in the nuclear sap is about 25%, and for mRNA in the cytoplasm 56%.