After a certain amount of time, the message is degraded into its component
nucleotides, usually with the assistance of RNases. The limited longevity of
mRNA enables a cell to alter protein synthesis rapidly in response to its changing
Different mRNAs within the same cell have distinct lifetimes. In bacterial
cells, individual mRNAs can survive from seconds to more than an hour; in mammalian
cells, mRNA lifetimes range from several minutes to days. The greater the stability
of an mRNA, the more protein may be produced from that transcript.
Ribonuclease, abbreviated commonly as RNase, is an nuclease that catalyzes the breakdown of RNA into smaller components. They can be divided into endonucleases and exonucleases, and comprise several sub-classes within the EC 3.1 class of enzymes.
RNases are extremely common, resulting in very short lifespans for any RNA that is not in a protected environment. One mechanism of protection is ribonuclease inhibitor (RI), which comprises a relatively large fraction of cellular protein (~0.1%) and which binds to certain ribonucleases with the highest affinity of any protein-protein interaction; the dissociation constant for the RI-RNase A complex is ~20 fM under physiological conditions. RI is used in most laboratories that study RNA to protect their samples against degradation from environmental RNases.
Perhaps surprisingly, RNases tend to be insensitive to the cleaved sequence. There appear to be no RNase analogs of the restriction enzymes, which cleave highly specific sequences of double-stranded DNA. This deficit may be overcome using RNase H and single-stranded DNA complementary to the desired cleavage sequence.
RNases play a critical role in many biological processes, including angiogenesis and self-incompatibility in flowering plants (angiosperms).
Major types of endoribonucleases
- RNase A is an RNase that is commonly used in research. RNase A (e.g., bovine pancreatic ribonuclease A: PDB 2AAS, EC 220.127.116.11) is one of the hardiest enzymes in common laboratory usage; one method of isolating it is to boil a crude cellular extract until all enzymes other than RNase A are denatured. It is sequence specific for single stranded RNAs. It cleaves 3'end of unpaired C and U residues.
- RNase P is a type of ribonuclease and is currently under heavy research. RNase P is unique from other RNases in that it is a ribozyme - a ribonucleic acid that acts as a catalyst in the same way that a protein based enzyme would. Its function is to cleave off an extra, or precursor, sequence of RNA on tRNA molecules. Further RNase P is one of two known multiple turnover ribozymes in nature (the other being the ribosome).
- RNase H is a ribonuclease that cleaves the RNA in a DNA/RNA duplex to produce ssDNA. RNase H is a non-specific endonuclease and catalyzes the cleavage of RNA via a hydrolytic mechanism, aided by an enzyme-bound divalent metal ion. In contrast to other ribonucleases, such as RNase V1, RNase H leaves a 3'-phosphorylated product.
- RNase III is specific for double-stranded RNA.
- RNase T1 is sequence specific for single stranded RNAs. It cleaves 3'-end of unpaired G residues.
- RNase T2 is sequence specific for single stranded RNAs. It cleaves 3'-end of all 4 residues, but preferentially 3'-end of As.
- RNase U2 is sequence specific for single stranded RNAs. It cleaves 3'-end of unpaired A residues.
- RNase V1 is non-sequence specific for double stranded RNAs. It cleaves base-paired nucleotide residues.
- RNase I cleaves 3'-end of of all 4 residues with no base preference
- RNase PhyM is sequence specific for single stranded RNAs. It cleaves 3'-end of unpaired A and U residues.
- RNase V
Major types of exoribonucleases
- Polynucleotide Phosphorylase (PNPase) functions both as an exonuclease as well as a nucleotidyltransferase.
- RNase PH functions both as an exonuclease as well as a nucleotidyltransferase.
- RNase II is responsible for the processive 3'-to-5' degradation of single-stranded RNA.
- RNase R is a close homolog of RNase II, but it can, unlike RNase II, degrade RNA with secondary structures without help of accessory factors.
- RNase D is involved in the 3'-to-5' processing of pre-tRNAs.
- RNase T is the major contributor for the 3'-to-5' maturation of many stable RNAs.
- Oligoribonuclease degrades short oligonucleotides to mononucleotides.
- Exoribonuclease I degrades single-sranded RNA from 5'-to-3', exists only in eukaryotes.
- Exoribonuclease II is a close homolog of Exoribonuclease I.
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