These proteins are highly similar to each other but have distinct functions. We showed that AtXRN2, but not AtXRN3, has an unexpected role in pre-rRNA processing at the early endonucleolytic cleavage at site P that is carried out by an unknown component of a large U3-containing NF D ribonucleoprotein complex. Our data reveal that AtXRN2 performs initial shortening of the 5’ external pre-rRNA spacer prior to the cleavage, and this trimming is required to expose the site for processing. AtXRN2 also degrades polyadenylated rRNA precursors and excised pre-rRNA spacer fragments, contributing to the nuclear polyadenylation-dependent RNA surveillance pathway.
