ALTERNATIVE 3'-END PROCESSING OF U5 SNRNA BY RNASE-III

The cellular components required to form the 3' ends of small nuclear RNAs are unknown. U5 snRNA from Saccharomyces cerevisiae is found in t wo forms that differ in length at their 3' ends (U5L and U5S). When ad ded to a yeast cell free extract, synthetic pre-U5 RNA bearing downstr eam genomic sequences is processed efficiently and accurately to gener ate both mature forms of U5. The two forms of U5 are produced in vitro by alternative 3'-end processing. A temperature-sensitive mutation in the RNT1 gene encoding RNase III blocks accumulation of U5L in vivo. In vitro, alternative cleavage of the U5 precursor by RNase III determ ines the choice between the two multistep pathways that lead to U5L an d U5S, one of which (U5L) is strictly dependent on RNase III. These re sults identify RNase III as a trans-acting factor involved in 3'-end f ormation of snRNA and show how RNase III might regulate alternative RN A processing pathways.