endoribonucleases

Endoribonucleases are enzymes that cleave RNA at internal sites, generating fragments with new 5' and 3' ends. This internal scission distinguishes them from exoribonucleases, which remove nucleotides from RNA termini. Endoribonucleases participate in RNA processing, maturation, turnover, and defense, and many require divalent metal ions or operate as components of ribonucleoprotein complexes.

Endoribonucleases vary in specificity and mechanism. Some are sequence- or structure-specific, recognizing particular motifs or RNA duplexes, while others act more broadly. Their catalytic strategies commonly involve positioning a water molecule for nucleophilic attack on the phosphodiester backbone, with metal ions or catalytic residues facilitating the reaction. They may target single-stranded RNA, double-stranded RNA, or structured precursors, reflecting diverse roles in RNA metabolism.

Major families and examples include: RNase A family, soluble enzymes that preferentially cleave after pyrimidines in single-stranded RNA; RNase III family, which processes double-stranded RNA and includes enzymes such as Drosha and Dicer involved in microRNA and siRNA pathways; RNase P, a ribonucleoprotein complex that trims leader sequences from precursor tRNAs; and bacterial RNases such as RNase E and RNase G that participate in general mRNA turnover. In eukaryotes, RNase L participates in antiviral defense by cleaving viral and cellular RNAs, and Argonaute proteins in the RNA-induced silencing complex execute endoribonucleolytic cleavage of target mRNAs. CRISPR-associated Cas13 proteins provide programmable RNA-targeting endoribonuclease activity in certain immune systems.

Biological roles include gene-expression regulation, RNA maturation, quality control, and defense against pathogens. In biotechnology, endoribonucleases underlie RNA interference, CRISPR-based diagnostics, and various RNA-processing technologies.