Alternative 3' processing of Xenopus alpha-tubulin mRNAs; efficient use of a CAUAAA polyadenylation signal.

The Xenopus laevis alpha-tubulin gene X alpha T14 produces two mRNAs of 1.7 and 2.15 kb and we have shown that this is due to the use, at approximately equal frequency, of alternative 3' processing sites. Unusually, the hexanucleotide polyadenylation signal responsible for use of the downstream site, pA2, is CAUAAA in contrast to the consensus AAUAAA used at the upstream site, pA1. Since such a variant hexanucleotide would normally be expected to reduce drastically the efficiency of 3' processing, we have examined the 3' flanking sequences involved in pA2 usage in injected oocytes. In deletion mutants with 40 bp or 440 bp of 3' flanking DNA use of pA2 was almost totally abolished whereas when 770 bp of the natural flank was present pA2 was used normally. This polyadenylation signal therefore requires an unexpectedly large amount of flanking DNA and we have identified in the required region a member of a novel family of 450 bp interspersed repeats that we have termed Pir elements. We speculate that because of the variant hexanucleotide efficient use of pA2 has to be potentiated by the Pir element, perhaps through an effect on transcriptional pausing or termination.