Polyadenylation of rRNA- and tRNA-based yeast transcripts cleaved by internal ribozyme activity.

Polyadenylation, an important step in 3' end-processing of mRNA in eukaryotes, results in a poly(A) tail that ensures RNA transport into the cytoplasm and subsequent translation. Addition of a poly(A) tail is restricted to transcripts that are synthesized by RNA polymerase II. Here, we demonstrate that the 3' ends of yeast transcripts based on rRNA and tRNA, respectively, can be polyadenylated in vivo. The transcripts were modified by insertion of a self-cleaving hammerhead ribozyme sequence in the corresponding gene. Both the rDNA-based transcript and the tRNA transcript were cleaved efficiently by the hammerhead ribozyme, resulting in two stable cleavage products. The 5' cleavage product was found to be polyadenylated in both cases. This demonstrates that, in yeast, transcripts that are usually synthesized by RNA polymerase I or III can be polyadenylated if the 3' end of the transcript has been generated independently by a ribozyme.