Ribonuclease T (''RNase T'', ''exonuclease T'', ''exo T'') is a

ribonuclease Ribonuclease (commonly abbreviated RNase) is a type of nuclease that catalyzes the degradation of RNA into smaller components. Ribonucleases can be divided into endoribonucleases and exoribonucleases, and comprise several sub-classes within th ...

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

involved in the maturation of

transfer RNA Transfer ribonucleic acid (tRNA), formerly referred to as soluble ribonucleic acid (sRNA), is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length (in eukaryotes). In a cell, it provides the physical link between the gene ...

and

ribosomal RNA Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribosomal ...

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

, as well as in

DNA repair DNA repair is a collection of processes by which a cell (biology), cell identifies and corrects damage to the DNA molecules that encode its genome. A weakened capacity for DNA repair is a risk factor for the development of cancer. DNA is cons ...

pathways. It is a member of the DnaQ family of

exonuclease Exonucleases are enzymes that work by cleaving nucleotides one at a time from the end (exo) of a polynucleotide chain. A hydrolyzing reaction that breaks phosphodiester bonds at either the 3′ or the 5′ end occurs. Its close relative is th ...

s and non- processively acts on the 3' end of single-stranded

nucleic acid Nucleic acids are large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a pentose, 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nuclei ...

s. RNase T is capable of cleaving both

DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...

and

RNA Ribonucleic acid (RNA) is a polymeric molecule that is essential for most biological functions, either by performing the function itself (non-coding RNA) or by forming a template for the production of proteins (messenger RNA). RNA and deoxyrib ...

, with extreme

sequence In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is cal ...

specificity discriminating against

cytosine Cytosine () (symbol C or Cyt) is one of the four nucleotide bases found in DNA and RNA, along with adenine, guanine, and thymine ( uracil in RNA). It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attac ...

at the 3' end of the

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (aquatic environment), the earthy material that exi ...

Structure and mechanism

RNAse T catalyzes the removal of nucleotides from the 3' end of both RNA and DNA. It is inhibited by both double stranded DNA and RNA, as well as

residues on the 3' end of RNA. Two cytosines at the 3' end of RNA appear to remove the activity of RNAse T entirely. This cytosine effect, however, is observed less with ssDNA. This lack of sequence specificity in ssDNA, combined with its ability to act on ssDNA close to a duplex region, has led to its use in creating blunt ends for DNA cloning. Structurally, RNAse T exists as an anti-parallel dimer and requires a divalent cation to function. RNAse T pi interactions

RNAse T is able to achieve its sequence specificity in RNA digestion via several

aromatic In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated system, conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected from conjugati ...

residues that sandwich between nucleobases. The π-π interactions between four

phenylalanine Phenylalanine (symbol Phe or F) is an essential α-amino acid with the chemical formula, formula . It can be viewed as a benzyl group substituent, substituted for the methyl group of alanine, or a phenyl group in place of a terminal hydrogen of ...

residues and the two nucleotides at the 3' end are different depending on the identify of the nucleotides, which changes the conformation and thus activity of the enzyme. An additional

glutamic acid Glutamic acid (symbol Glu or E; known as glutamate in its anionic form) is an α- amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can ...

residue rotates to hydrogen bond to cytosine but not other bases, further increasing specificity.

Function

A member of the larger DEDD family of exoribonucleases, RNAse T plays a key role in the maturation of tRNA as well as the maturation of the 5S and 23S rRNA domains. Specifically, RNAse T cleaves the 3' AMP residue from the 3' CCA sequences at the end of tRNA, which explains RNAse T's sequence specificity for stopping at the 3' CC sequence. Additionally, RNAse T can play a role in DNA repair by cleaving the 3' end of bulge DNA. While E. coli can survive without RNAse T, its absence leads to slower life cycles and weakened response to starvation. Additionally, the presence of RNAse T in E. coli is linked to increased resistance to UV damage. It has been theorized that, while other ribonculeases can perform the function RNAse T, the fact that RNAse T is more effective at cleaving DNA and RNA near double-stranded regions means that alternatives are less effective. Despite the apparent usefulness of RNAse T, the enzyme is only found in

gammaproteobacteria ''Gammaproteobacteria'' is a class of bacteria in the phylum ''Pseudomonadota'' (synonym ''Proteobacteria''). It contains about 250 genera, which makes it the most genus-rich taxon of the Prokaryotes. Several medically, ecologically, and scienti ...

. In E. coli, RNAse T is encoded by the rnt gene and is hypothesized to have diverged from the proofreading subunits of polymerase III during the emergence of gammaproteobacteria.

References

{{Portal bar, Biology, border=no Ribonucleases EC 3.1.13