= Exoribonuclease
An \b[exoribonuclease] is an <enzyme> that removes <nucleotide> units successively from an end of an <RNA> chain. The prefix exo describes end-directed action, in contrast to cleavage at an internal position. The attacked end must be specified: <RNase R> acts3′→5′, whereas <Xrn1> acts5′→3′.<How RNase R Degrades Structured RNA: Role of the Helicase Activity and the S1 Domain by Sk Tofajjen Hossain and others>[Abstract: RNase R3′ exoribonuclease]{ref}<Structural and biochemical studies of the 5′→3′ exoribonuclease Xrn1 by Jeong Ho Chang and others>[Abstract and Introduction: Xrn-family5′→3′ activity]{ref}
End removal has a different inventory from an internal cut. In an idealized 100-nucleotide chain, 20 successive single-nucleotide removal events leave an 80-nucleotide chain. One internal cut can split that same original chain into two fragments while retaining all 100 <nucleotides> in those fragments. A change in fragment size therefore needs interpretation alongside product amounts and the type of cleavage.
Direction is also distinct from <processivity>, the persistence of successive reactions during one binding encounter. Binding to an end, removing a <nucleotide> and remaining associated are separable parts of an enzyme's behavior. End accessibility, <RNA> structure and terminal chemistry can all affect which substrates are degraded.
The 2011 structural and biochemical study of <Xrn1> illustrates why the whole protein matters. Crystal structures of a defined construct were combined with mutational tests; regions remote from the <active site> helped stabilize its catalytic architecture. Thus a lost degradation signal after deleting a region need not mean that the deleted region itself contains the bond-cleaving chemistry.<Structural and biochemical studies of the 5′→3′ exoribonuclease Xrn1 by Jeong Ho Chang and others>[Abstract and Results: E178Q construct and roles of domainsD1–D4]{ref}
Back to article page
