exoRNases

ExoRNases, or exoribonucleases, are a class of enzymes that degrade RNA molecules by sequentially removing nucleotides from either the 3′ or 5′ end. These enzymes play critical roles in RNA processing, turnover, and regulation within cells. Unlike endonucleases, which cleave RNA internally, exoRNases act processively, breaking down RNA strands from one terminus to the other.

ExoRNases can be broadly categorized into two types based on their directionality: 3′→5′ exoribonucleases and 5′→3′ exoribonucleases. The 3′→5′ exoRNases, such as the eukaryotic exosome complex and bacterial RNase II, are the most common and are involved in RNA degradation, quality control, and the processing of non-coding RNAs. The 5′→3′ exoribonucleases, like bacterial RNase R and eukaryotic RNase R homologs, are less abundant but still essential for RNA turnover and stress responses.

In eukaryotes, the exosome—a multi-subunit complex—is a key player in RNA degradation, particularly in the nucleus and cytoplasm. It collaborates with other factors, such as poly(A) ribonuclease (PARN) and the nuclear exosome targeting (NEXT) complex, to degrade aberrant or unwanted RNAs. Defects in exosome function have been linked to diseases, including neurodegenerative disorders and cancer.

Prokaryotes also possess exoRNases, such as RNase II and RNase R, which contribute to RNA turnover, antibiotic stress responses, and the regulation of small RNAs. These enzymes often work alongside endonucleases to ensure efficient RNA degradation.

ExoRNases are also exploited in molecular biology for applications like RNA footprinting, where they help map protein-RNA interactions by selectively degrading exposed RNA regions. Their specificity and processive nature make them valuable tools in studying RNA metabolism and function.