Three conserved members of the RNase D family have unique and overlapping functions in the processing of 5S, 5.8S, U4, U5, RNase MRP and RNase P RNAsin yeast

The biogenesis of a number of RNA species in eukaryotic cells requires 3' p rocessing. To determine the enzymes responsible for these trimming events, we created yeast strains lacking specific 3' to 5' exonucleases. In this wo rk, we describe the analysis of three members of the RNase D family of exon ucleases (Rex1p, Rex2p and Rex3p), This work led to three important conclus ions. First, each of these exonucleases is required for the processing of d istinct RNAs, Specifically, Rex1p, Rex2p and Rex3p are required for 5S rRNA , U4 snRNA and MRP RNA trimming, respectively. Secondly, some 3' exonucleas es are redundant with other exonucleases. Specifically, Rex1p and Rex2p fun ction redundantly in 5.8S rRNA maturation, Rex1p, Rex2p and Rex3p are redun dant for the processing of U5 snRNA and RNase P RNA, and Rex1p and the exon uclease Rrp6p have an unknown redundant essential function. Thirdly, the de monstration that the Rex proteins can affect reactions that have been attri buted previously to the exosome complex indicates that an apparently simple processing step can be surprisingly complex with multiple exonucleases wor king sequentially in the same pathway.