The two similarly expressed genes encoding U3 snRNA in Schizosaccharomyces pombe lack introns

Both genes encoding U3 small nuclear RNA (snRNA) from the budding yeast Saccharomyces cerevisiae were recently shown to be interrupted by introns of the type removed by the pre-mRNA splicing machinery. We previously described one of the two U3 genes from the fission yeast Schizosaccharomyces pombe. In the present work, the second S. pombe U3 coding sequence was identified, and direct RNA sequence analysis was used to show that neither the U3A nor the U3B gene from this organism contains an intervening sequence. Our data also demonstrate that, as expected, the two RNAs exhibit great primary- and secondary-structure conservation. These similarities are not likely to be the result of a recent gene duplication or conversion event, because the DNA sequences flanking the U3A and U3B genes have diverged substantially. A notable exception is a 19-bp block, centered 36 nucleotides upstream from the transcriptional start site, in which the two loci match in 15 positions; this motif may represent an RNA polymerase II upstream regulatory element, because related sequences are found preceding fission yeast U1, U2, U4, and U5 snRNA genes. The significance of a short conserved sequence just downstream of the U3A and U3B genes is unknown, as it is not found 3' to other snRNA coding sequences in S. pombe. The 5' one-third of U3B RNA can be folded into a dual hairpin structure, as we previously proposed for Schizosaccharomyces pombe U3A and for other lower eukaryotic U3 homologues. Quantitation of fission yeast U3A and U3B indicates that, in contrast to snR17A and B in Saccharomyces cerevisiae, these RNAs accumulate to similar levels.