Core snRNP proteins bind snRNA through the conserved Sm site, PuA(U)(n≥3)GPu. While yeast U1 snRNA has three matches to the Sm consensus, the U1 3'-terminal Sm site was found to be both necessary and sufficient for U1 function. Mutation of this site inhibited pre-mRNA splicing, blocked cell division and resulted in the accumulation of two 3'-extended forms of the U1 snRNA. Cells which harbor the Sm site mutation lack mature U1 RNA (U1α) but have a minor polyadenylated species, U1γ, and a prominent, non-polyadenylated species, U1β. Metabolic depletion of the essential Sm core protein, Smd1p, also resulted in the increased accumulation of U1β and U1γ. In vitro, synthetic U1 precursors were cleaved by Rnt1p (RNase III) very near the U1β 3'-end observed in vivo. We propose that U1β is an Rnt1p-cleaved intermediate and that U1 maturation to the U1α form occurs through an Sm-sensitive step. Interestingly, both U1α and a second, much longer RNA, U1ε, were produced in an rnt1 mutant strain. These results suggest that yeast U1 snRNA processing may progress through Rnt1p-dependent and Rnt1p-independent pathways, both of which require a functional Sm site for final snRNA maturation.
|Number of pages||9|
|Journal||Nucleic Acids Research|
|State||Published - Jan 15 1999|
Bibliographical noteFunding Information:
We thank Manny Ares Jr, Guillaume Chanfreau, Tom Cech and Thoru Pederson for their helpful comments and discussions. We are indebted to Manny Ares Jr and Sherif Abou Elela for the MA52 and MA6 yeast, purified GST–Rnt1p and the RNase III control substrate plasmid. This work was supported by National Institutes of Health grant GM42476 to B.C.R. R.L.S. is a 1998–1999 American Association of University Women Postdoctoral Fellow.
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