Yeast U1 snRNA (568 nucleotides) is 3.5-fold larger than its mammalian counterpart (164 nucleotides) and contains apparent sequence homology only at the 5′ and 3′ ends. We have used deletion analysis to determine whether the yeast-specific U1 sequences play essential roles in vivo. Yeast cells carrying a deletion of more than 60% (355 nucleotides) of the single-copy U1 gene are viable, though slow-growing, while a deletion of 316 nucleotides allows essentially wild-type growth. The boundaries of the viable deletions define a dispensable internal domain which comprises sequences unique to yeast. In contrast, the essential 5′ and 3′ terminal domains correspond to phylogenetically conserved sequences and/or structures previously implicated in RNA:RNA and RNA:protein interactions. The minimal essential sequences of yeast U1 can be drawn in a secondary structure which resembles metazoan U1 in four of seven structural domains.
Bibliographical noteFunding Information:
We thank J. Umen, M. Haltiner Jones, and D. Frank for carefully reading the manuscript. This work was supported by grants from the NIH (GM21119) to C.G. and from the NSF (DBM 9007430) to P.G.S.