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 RNAs in yeast

The biogenesis of a number of RNA species in eukaryotic cells requires 3′ processing. To determine the enzymes responsible for these trimming events, we created yeast strains lacking specific 3′ to 5′ exonucleases. In this work, we describe the analysis of three members of the RNase D family of exonucleases (Rex1p, Rex2p and Rex3p). This work led to three important conclusions. First, each of these exonucleases is required for the processing of distinct RNAs. Specifically, Rex1p, Rex2p and Rex3p are required for 5S rRNA, U4 snRNA and MRP RNA trimming, respectively. Secondly, some 3′ exonucleases are redundant with other exonucleases. Specifically, Rex1p and Rex2p function redundantly in 5.8S rRNA maturation, Rex1p, Rex2p and Rex3p are redundant for the processing of U5 snRNA and RNase P RNA, and Rex1p and the exonuclease Rrp6p have an unknown redundant essential function. Thirdly, the demonstration that the Rex proteins can affect reactions that have been attributed previously to the exosome complex indicates that an apparently simple processing step can be surprisingly complex with multiple exonucleases working sequentially in the same pathway.