We have used comparative sequence analysis and deletion analysis to ex
amine the secondary structure of the U5 small nuclear RNA (snRNA), an
essential component of the pre-mRNA splicing apparatus. The secondary
structure of Saccharomyces cerevisiae U5 snRNA was studied in detail,
while sequences from six other fungal species were included in the phy
logenetic analysis. Our results indicate that fungal U5 snRNAs, like t
heir counterparts from other taxa, can be folded into a secondary stru
cture characterized by a highly conserved stem-loop (stem-loop 1) that
is flanked by a moderately conserved internal loop (internal loop 1).
In addition, several of the fungal U5 snRNAs include a novel stem-loo
p structure (ca. 30 nucleotides) that is adjacent to stem-loop 1. By d
eletion analysis of the S. cerevisiae snRNA, we have demonstrated that
the minimal U5 snRNA that can complement the lethal phenotype of a U5
gene disruption consists of (i) stem-loop 1, (ii) internal loop 1, (i
ii) a stem-closing internal loop 1, and (iv) the conserved Sm protein
binding site. Remarkably, all essential, U5-specific primary sequence
elements are encoded by a 39-nucleotide domain consisting of stem-loop
1 and internal loop 1. This domain must, therefore, contain all US-sp
ecific sequences that are essential for splicing activity, including b
inding sites for U5-specific proteins.