FUNCTIONAL MAPPING OF THE U3 SMALL NUCLEOLAR RNA FROM THE YEAST SACCHAROMYCES-CEREVISIAE

The U3 small nucleolar RNA participates in early events of eukaryotic pre-rRNA cleavage and is essential for formation of 18S rRNA, Using an in vivo system, we have developed a functional map of the U3 small nu cleolar RNA from Saccharomyces cerevisiae, The test strain features a galactose-dependent U3 gene in the chromosome and a plasmid-encoded al lele with a unique hybridization tag, Effects of mutations on U3 produ ction were analyzed by evaluating RNA levels in cells grown on galacto se medium, and effects on U3 function were assessed by growing cells o n glucose medium, The major findings are as follows: (i) boxes C' and D and flanking helices are critical for U3 accumulation; (ii) boxes B and C are not essential for U3 production but are important for functi on, most likely due to binding of a trans-acting factor(s); (iii) the 5' portion of U3 is required for function but not stability; and, (iv) strikingly, the nonconserved hairpins 2, 3, and 4, which account for 50% of the molecule, are not required for accumulation or function.