exonucleolytic

Exonucleolytic refers to enzymatic activities that remove successive nucleotides from the ends of nucleic acid polymers, such as DNA or RNA. Exonucleases differ from endonucleases in that they act at terminal regions rather than within the chain, releasing nucleotides one by one. Exonucleolytic activity can proceed in either direction: 3' to 5' exonucleases remove nucleotides from the 3' end toward the 5' end, whereas 5' to 3' exonucleases remove from the 5' end toward the 3' end.

Biochemically, exonucleases commonly require divalent metal ions such as magnesium or manganese to catalyze phosphodiester bond cleavage. Activity ranges from highly processive to distributive, depending on the enzyme. Substrate context matters: some exonucleases target single-stranded regions, others act on double-stranded DNA or RNA, and some preferentially degrade through overhangs or mismatches.

In biology, exonucleolytic activity supports several essential processes. Three broad contexts are prominent: proofreading during DNA replication, where 3' to 5' exonucleases remove misincorporated nucleotides; participation in DNA repair pathways that trim damaged or mismatched termini; and RNA metabolism, including processing, maturation, and degradation of transcripts. Exonucleolytic functions are carried out by a variety of enzymes and complexes, such as specialized ribonucleases and multi-subunit exosome complexes that perform 3' to 5' RNA decay, as well as cytoplasmic mechanisms that degrade RNA from the 5' end.

In laboratory settings, exonucleases are used to trim or blunt DNA ends, remove primers, and generate defined termini for cloning, sequencing, or library preparation. Selection of an exonuclease depends on directionality, substrate preference, and sensitivity to inhibitors.