THE RIBOSOMAL-RNA-PROCESSING FUNCTION OF THE YEAST U14 SMALL NUCLEOLAR RNA CAN BE RESCUED BY A CONSERVED RNA HELICASE-LIKE PROTEIN

The phylogenetically conserved U14 small nucleolar RNA is required for professing of rRNA, and this function involves base pairing with cons erved complementary sequences in 18S RNA. With a view to identifying o ther important U14 interactions, a stem-loop domain required for activ ity of Saccharomyces cerevisiae U14 RNAs (the Y domain) was first subj ected to detailed mutational analysis, The mapping results showed that most nucleotides of the Y domain can be replaced without affecting fu nction, except for loop nucleotides conserved among five different yea st species. Defective variants were then used to identify both intrage nic and extragenic suppressor mutations, All of the intragenic mutatio ns mapped within six nucleotides of the primary mutation, suggesting t hat suppression involves a change in conformation and that the loop el ement is involved in an essential intermolecular interaction rather th an intramolecular base pairing. A high-copy extragenic suppressor gene , designated DBP4 (DEAD box protein Jj, encodes an essential, putative RNA helicase of the DEAD-DEXH box family. Suppression by DBP4 (initia lly CA4 [T.-H. Chang, J, Arenas, and J, Abelson, Proc, Natl. Acad. Sci , USA 57:1571-1575, 1990]) restores the level of 18S rRNA and is speci fic for the Y domain but is not allele specific, DBP4 is predicted to function either in assembly of the U14 small nucleolar RNP or, more li kely, in its interaction with other components of the rRNA processing apparatus, Mediating the interaction of U14 with precursor 18S RNA is an especially attractive possibility.