The cleavage/polyadenylation activity triggered by a U-rich motif sequence is differently required depending on the poly(A) site location at either the first or last 3'-terminal exon of the 2'-5' oligo(A) synthetase gene.

Production of the two mRNAs encoding distinct forms of 2'-5'-oligoadenylate synthetase depends on processing that involves the recognition of alternative poly(A) sites and an internal 5'-splice site located within the first 3'-terminal exon. The resulting 1.6- and 1.8-kb mRNAs are expressed in fibroblast cell lines, whereas lymphoblastoid B cells, such as Daudi, produce only the 1.8-kb mRNA. In the present study, we have shown that the 3'-end processing at the last 3'-terminal exon occurs independently of the core poly(A) site sequence or the presence of regulatory elements. In contrast, in Daudi cells, the recognition of the poly(A) site at the first 3'-terminal exon is impaired because of an unfavorable sequence context. The 3'-end processing at this particular location requires a strong stabilization of the cleavage/polyadenylation factors, which can be achieved by the insertion of a 25-nucleotide long U-rich motif identified upstream of the last poly(A) site. Consequently, we speculate that in cells expressing the 1.6-kb mRNA, such as fibroblasts, direct or indirect participation of a specific mechanism or cell type-specific factors are required for an efficient polyadenylation at the first 3'-terminal exon.