Rat1 and Xrn2: The Diverse Functions of the Nuclear Rat1/Xrn2 Exonuclease.

The role of the nucleus of a eukaryotic cell during gene expression is not only limited to transcription and RNA processing but also includes the initial stages of RNA surveillance. All of these processes, and more precisely, transcription elongation and termination, 5'-end RNA maturation, and the removal of processing intermediates and aberrant molecules, require the activity of the nuclear 5'-3' exoribonuclease Rat1/Xrn2. This protein, together with its cytoplasmic counterpart, Xrn1, constitutes a highly conserved eukaryotic family of nucleases, whose roles exceed participation in RNA metabolism alone. Despite many years of extensive research and recent findings related to the structure and function of these enzymes revealed almost every year, several aspects are yet to be discovered.