exoribonucleases

Exoribonucleases are a class of ribonucleases that catalyze the removal of nucleotides from the ends of RNA molecules, degrading RNA from either the 5' end or the 3' end. They differ from endoribonucleases, which cleave phosphodiester bonds within the RNA strand. Exoribonucleases are categorized by directionality: 5'-to-3' exoribonucleases remove nucleotides from the 5' end, while 3'-to-5' exoribonucleases act from the 3' end.

In eukaryotes, prominent examples include the 5'-to-3' exoribonucleases XRN1 in the cytoplasm and XRN2 in the nucleus. 3'-to-5' exoribonuclease activity is provided mainly by the exosome complex, whose catalytic subunits include DIS3 (Rrp44) and its nuclear counterpart Rrp6 (DIS3L). In bacteria and archaea, exoribonuclease activities are carried by enzymes such as RNase J (which can function in a 5'-to-3' direction in some organisms) and RNases II/RNase R, which perform 3'-to-5' degradation. The exact composition and directionality can vary across domains of life.

Functions and significance: Exoribonucleases participate in RNA processing and surveillance, including maturation of rRNA, tRNA, and small nuclear/cersonal RNAs, as well as mRNA decay and quality control. They trim or remove defective transcripts, regulate RNA turnover, and help maintain cellular RNA homeostasis under stress. Many exoribonucleases require divalent metal ions, such as Mg2+ or Mn2+, and can be highly processive, releasing nucleoside monophosphates as they progress.

Regulation and context: Some exoribonucleases operate within multi-protein complexes, coordinating processing with other RNA metabolic activities. Dysfunction or misregulation of exoribonucleases is linked to cellular stress responses and disease, and these enzymes are subjects of ongoing research in RNA metabolism and antiviral strategies.