The sequence of U3 fromSchizosaccharomyces pombesuggests structural divergence of this snRNA between metazoans and unicellular eukaryotes

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Abstract
We have cloned and sequenced one of the two genes encoding a 255 nucleotide small nuclear RNA from the fission yeast Schizosaccharomyces pombe . Based on the presence of four regions of primary sequence conservation and a predicted secondary structure similar to that previously proposed for human U3, we conclude that this molecule is the fission yeast homologue of this mammalian snRNA. The 5' one-third of fission yeast U3 is, however, unable to form a single stable hairpin as proposed for this region of the human RNA, but rather folds into two stem-loop structures. By analogy to fission yeast U3, we propose revised secondary structures containing two hairpins for this portion of the U3-like snRNAs from Saccharomyces cerevisiae and Dictyostelium discoideum . Thus, our data suggest that the structure of U3 snRNA has diverged in lower and higher eukaryotes.