HOMOLOGOUS MESSENGER-RNA 3' END FORMATION IN FISSION AND BUDDING YEAST

Sequences resembling polyadenylation signals of higher eukaryotes are present downstream of the Schizosaccharomyces pombe ura4+ and cdc10+ coding regions and function in HeLa cells. However, these and other mammalian polyadenylation signals are inactive in S.pombe. Instead, we find that polyadenylation signals of the CYC1 gene of budding yeast Saccharomyces cerevisiae function accurately and efficiently in fission yeast. Furthermore, a 38 bp deletion which renders this RNA processing signal non-functional in S.cerevisiae has the equivalent effect in S.pombe. We demonstrate that synthetic pre-mRNAs encoding polyadenylation sites of S.pombe genes are accurately cleaved and polyadenylated in whole cell extracts of S.cerevisiae. Finally, as is the case in S.cerevisiae, DNA sequences encoding regions proximal to the S.pombe mRNA 3' ends are found to be extremely AT rich; however, no general sequence motif can be found. We conclude that although fission yeast has many genetic features in common with higher eukaryotes, mRNA 3' end formation is significantly different and appears to be formed by an RNA processing mechanism homologous to that of budding yeast. Since fission and budding yeast are evolutionarily divergent, this lower eukaryotic mechanism of mRNA 3' end formation may be generally conserved.