The sequence of U3 from Schizosaccharomyces pombe suggests structural divergence of this snRNA between metazoans and unicellular eukaryotes.
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.
Porter GL, Brennwald PJ, Holm KA, Wise JA