Ribonuclease

An enzyme that cleaves RNA. Ribonuclease A is one example.

Some ribonucleases cut internal phosphodiester bonds, whereas others remove nucleotides from an end. Selective RNA degradation contributes to processing and regulation, so cleavage can produce a functional mature RNA as well as destroy a transcript. ^[ref]

An 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′.^[ref][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.^[ref]

Polynucleotide phosphorylase is an enzyme that can remove nucleotides from the $3^{\prime }$ end of RNA by phosphorolysis, releasing nucleoside diphosphates. Its reversible reaction is conventionally writtenwhere ${\mathrm{P}}_{\mathrm{i}}$ is inorganic phosphate and $\mathrm{NDP}$ is the released nucleoside diphosphate. The accepted enzyme classification is EC2.7.7.8.^[ref]

For a sequence-accounting example, start with $5^{\prime }$-ACGU-$3^{\prime }$. One terminal cleavage removes the U unit as uridine diphosphate, leaving ACG. Removing the next unit releases the corresponding G-containing diphosphate. The direction is $3^{\prime }$ toward $5^{\prime }$; phosphate participation and the diphosphate product distinguish this chemistry from hydrolytic cleavage releasing a nucleoside monophosphate.

Under conditions favoring the reverse reaction, nucleoside diphosphates can supply units for chain extension. The reversible reaction itself contains no complementary template, so synthetic activity alone does not establish the sequence-copying mechanism of an RNA polymerase.^[ref]

A 2000 X-ray crystallography study revealed a three-subunit assembly with a central channel. The structure contained a phosphate analogue at catalytic sites but no bound RNA. The proposed connection between the channel, RNA handling and processivity was therefore a structural model, rather than a direct movie of a chain moving through the enzyme.^[ref]

The polynucleotide phosphorylase central channel is the passage through the ring-shaped protein trimer formed by polynucleotide phosphorylase. It helps organize RNA access to catalytic regions of the assembly. The channel is a property of the assembled enzyme, rather than a hollow tube inside one isolated polypeptide.^[ref]

Different experiments supply different levels of evidence. The 2000 structure contained a phosphate analogue but no bound RNA and proposed a channel role in processivity. A 2021 cryo-electron microscopy study captured an RNA-containing state of a mycobacterial enzyme, including a terminal oligonucleotide in an active site and an adenine base in the central tunnel. That directly places substrate features in the assembly without resolving every segment of a complete chain.^[ref]

A conceptual geometry test illustrates the importance of routing. Imagine an RNA with a flexible single-stranded tail followed by a bulky paired region. A channel admitting the tail need not admit that paired region. Entry, retention and bond cleavage can therefore impose different constraints on the observed reaction, even when the chemical active site is unchanged.

The structural observation should not be interpreted as a molecular movie. Mobility, unresolved density and transitions between catalytic states require biochemical or kinetic evidence alongside the static structure. In particular, a ring-shaped assembly alone does not prove unlimited processivity.

Xrn1 is an exoribonuclease that degrades RNA from its $5^{\prime }$ end toward its $3^{\prime }$ end. It hydrolyzes successive phosphodiester bonds, releasing nucleoside monophosphates. Xrn1 participates in cytoplasmic messenger RNA turnover, and terminal chemistry helps determine which substrates it can attack.^[ref]

A short sequence makes the direction concrete. For $5^{\prime }$-ACGU-$3^{\prime }$, initial removal of A leaves CGU. By contrast, polynucleotide phosphorylase acting at the other end first removes U. An internal cut and sequential end removal therefore leave different intermediate patterns even if both eventually destroy the same original molecule.

A substrate-complex study identified a pocket recognizing the terminal $5^{\prime }$ monophosphate. The crystallized complex used a truncated, catalytically impaired protein with a short DNA oligonucleotide; biochemical assays separately tested RNA degradation. That distinction matters when connecting a static structure to the behavior of an active enzyme.^[ref]

The same study tested defined RNA–DNA hybrids with different single-stranded RNA overhangs. Degradation through the paired region occurred with five- and eight-nucleotide $5^{\prime }$ extensions but failed with a two-nucleotide extension under the tested conditions. This supports coupling of hydrolysis to strand separation after productive end engagement, without claiming that every structured RNA is equally accessible.^[ref]

End chemistry, structure and processivity thus describe separate constraints. A terminal monophosphate can favor recognition while an inaccessible end or an obstructing structure still limits degradation.

Xrn1 5′-phosphate recognition is the selective interaction between Xrn1 and the monophosphate at an RNA chain's $5^{\prime }$ end. The enzyme recognizes the terminal nucleotide in context; free inorganic phosphate elsewhere in the sample is not the same substrate feature.

In a structural study of a defined Xrn1 construct, a pocket containing Lys93, Gln97, Arg100 and Arg101 contacted the terminal phosphate, while His41 stacked with the terminal base. Mutating phosphate-pocket residues impaired cleavage and processivity, linking a structural contact to enzyme function.^[ref]

For a matched-substrate example, compare two RNAs with the same sequence but a $5^{\prime }$ monophosphate on one and a $5^{\prime }$ hydroxyl on the other. A difference in reaction progress can then test terminal chemistry while sequence is held fixed. Concentration and turnover regime must also be controlled: the study found that ordinary Xrn1 could degrade hydroxylated RNA under its single-turnover conditions, so monophosphate preference should not be rewritten as an absolute prohibition under every assay condition.^[ref]

Recognition also matters after an initial cleavage. The newly exposed end must adopt a productive configuration for the next step. Thus a change at a binding pocket can alter repeated action as well as initial substrate selection; a binding signal alone does not establish productive hydrolysis.

RNase R is a bacterial exoribonuclease that degrades RNA from the3′ end toward the5′ end. It hydrolyzes successive phosphodiester bonds rather than requiring an internal cut at each step. Its ability to digest structured RNA distinguishes it from enzymes restricted mainly to accessible single-stranded regions.^[ref]

The direction matters. For a sufficiently accessible linear RNA whose final three nucleotides are written5′–…GCA–3′, degradation encounters the terminal A before the preceding C. This notation illustrates direction, not a claim that an isolated three-nucleotide fragment is an efficient substrate. An intact circular RNA has no free end for this entry route; a strand break creates ends and changes the substrate's topology.

A2016 mechanistic study compared designed single-stranded and duplex RNA substrates with altered enzyme variants. It examined the S1 RNA-binding domain, intrinsic RNA helicase activity, ATP binding and conformational changes measured through tryptophan fluorescence. Those comparisons separated RNA binding, strand separation and phosphodiester cleavage rather than treating disappearance of an RNA band as a complete mechanistic explanation.^[ref]

Substrate structure and end accessibility therefore matter when interpreting resistance to digestion. Surviving RNA is an observation under the chosen reaction conditions; by itself it does not establish the exact structure or topology of every surviving molecule.

Ribonuclease T1 is a guanosine-selective ribonuclease: it cleaves susceptible phosphodiester bonds on the 3′ side of guanosine residues in single-stranded RNA. Its two-stage chemistry first creates an RNA cyclic phosphate end and a 5′-hydroxyl end, then converts the cyclic end to a 3′-phosphate end by hydrolysis.^[ref]

The current classification is EC 4.6.1.24, a lyase entry. The former EC 3.1.27.3 entry was transferred in 2020 because the primary cleavage step is a phosphorus–oxygen lyase reaction. Merely describing the final overall reaction as hydrolysis hides that first step.^[ref]

An invented sequence example makes its selectivity concrete. Read ACGUAAGCU from 5′ to 3′ and suppose all eligible internal bonds are accessible and completely cleaved. Cutting after each G gives ACG|UAAG|CU: fragments of three, four and two nucleotides, respectively. The nine nucleotides are redistributed among three fragments rather than disappearing. For a linear chain, $k$ distinct internal cuts produce $k+1$ fragments.

The letters alone do not guarantee that a folded RNA exposes every candidate bond. Sequence preference and physical accessibility jointly affect a cleavage pattern; the toy calculation explicitly assumes complete accessibility.

This selectivity also made ribonuclease T1 useful in historical sequence comparisons. A 1977 study characterized 16S ribosomal RNA from ten methane-producing microbial species through the fragments produced by T1 digestion, then compared the resulting catalogs.^[ref] Selective cleavage converted a large molecule into a collection of experimentally comparable sequence pieces.

Ribonuclease H cleaves the RNA component of an RNA-DNA hybrid, a paired structure containing one RNA strand and one DNA strand. Its substrate recognition therefore concerns both chemical composition and pairing, rather than merely the presence of RNA anywhere in a sample.^[ref]

A 1973 calf-thymus study purified the enzyme more than three thousandfold and tested several substrates. It cleaved an artificial DNA/RNA polymer pair and a hybrid made with phage DNA, while the tested ribosomal RNA was not attacked. Activity also required a suitable metal ion.^[ref]

The comparison illustrates a specificity test. Detecting cleavage in one preparation would show activity, while testing alternative substrates helps establish what the activity recognizes. Conversely, failure to cleave a hybrid under conditions lacking an activating ion would not establish that the hybrid is intrinsically resistant. Protein purification, substrate controls and assay conditions jointly support the molecular interpretation; the behavior of one preparation should not be assumed to specify every member of the broader enzyme family.

A nuclease participating in primary piRNA processing in Drosophila, distinct from the Dicer cleavage pathway.

An enzyme that processes appropriate double-stranded RNA substrates into small guide-RNA precursors in several RNA interference pathways.

A Dicer-like protein that processes canonical precursors into 24-nucleotide siRNAs.

The products contribute to canonical RNA-directed DNA methylation in plants. Processing converts a longer double-stranded precursor into guide-sized fragments, linking RNA production to later sequence-directed chromatin regulation. ^[ref]