An exoribonuclease is an exonuclease

Exonuclease

Exonucleases are enzymes that work by cleaving nucleotides one at a time from the end of a polynucleotide chain. A hydrolyzing reaction that breaks phosphodiester bonds at either the 3’ or the 5’ end occurs. Its close relative is the endonuclease, which cleaves phosphodiester bonds in the middle ...

 ribonuclease

Ribonuclease

Ribonuclease 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 the EC 2.7 and 3.1 classes of enzymes.-Function:All organisms studied contain...

, which are enzymes that degrade RNA

RNA

Ribonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....

 by removing terminal nucleotide

Nucleotide

Nucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...

s from either the 5' end or the 3' end of the RNA molecule. Enzymes that remove nucleotides from the 5' end are called 5'-3' exoribonucleases, and enzymes that remove nucleotides from the 3' end are called 3'-5' exoribonucleases.

Exoribonucleases can use either water to cleave the nucleotide-nucleotide bond (which is called hydrolytic activity) or inorganic phosphate (which is called phosphorolytic activity). Hydrolytic exoribonucleases are classified under EC number

EC number

The Enzyme Commission number is a numerical classification scheme for enzymes, based on the chemical reactions they catalyze....

 3.1 and phosphorolytic exoribonucleases under EC number 2.7.7. As the phosphorolytic enzymes use inorganic phosphate to cleave bonds they release nucleotide disphosphates

Adenosine diphosphate

Adenosine diphosphate, abbreviated ADP, is a nucleoside diphosphate. It is an ester of pyrophosphoric acid with the nucleoside adenosine. ADP consists of the pyrophosphate group, the pentose sugar ribose, and the nucleobase adenine....

), whereas the hydrolytic enzymes (which use water) release nucleotide monosphosphates

Adenosine monophosphate

Adenosine monophosphate , also known as 5'-adenylic acid, is a nucleotide that is used as a monomer in RNA. It is an ester of phosphoric acid and the nucleoside adenosine. AMP consists of a phosphate group, the sugar ribose, and the nucleobase adenine...

).

Exoribonucleases exist in all kingdoms of life, the bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, archaea

Archaea

The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

, and eukaryote

Eukaryote

A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...

s. Exoribonucleases are involved in the degradation of many different RNA species, including messenger RNA

Messenger RNA

Messenger RNA is a molecule of RNA encoding a chemical "blueprint" for a protein product. mRNA is transcribed from a DNA template, and carries coding information to the sites of protein synthesis: the ribosomes. Here, the nucleic acid polymer is translated into a polymer of amino acids: a protein...

, transfer RNA

Transfer RNA

Transfer RNA is an adaptor molecule composed of RNA, typically 73 to 93 nucleotides in length, that is used in biology to bridge the three-letter genetic code in messenger RNA with the twenty-letter code of amino acids in proteins. The role of tRNA as an adaptor is best understood by...

, ribosomal RNA

Ribosomal RNA

Ribosomal ribonucleic acid is the RNA component of the ribosome, the enzyme that is the site of protein synthesis in all living cells. Ribosomal RNA provides a mechanism for decoding mRNA into amino acids and interacts with tRNAs during translation by providing peptidyl transferase activity...

 and miRNA

Mirna

Mirna may refer to:geographical entities* Mirna , a river in Istria, Croatia* Mirna , a river in Slovenia, tributary of the river Sava* Mirna , a settlement in the municipality of Mirna in Southeastern Sloveniapeople...

. Exoribonucleases can be single proteins (like RNase D

RNase D

RNase D is one of the seven exoribonucleases identified in E. coli. It is an 3'-5' exoribonuclease and which has been shown to be involved in the 3' processing of various stable RNA molecules ; It is known to add the 3' CCA sequence to tRNA in prokaryotic tRNA processing. RNase D has homologues...

 or RNase PH

RNase PH

RNase PH is an 3'-5' exoribonuclease and nucleotidyltransferase, present in archaea and bacteria, that is involved in tRNA processing. Contrary to hydrolytic enzymes, it is a phosphorolytic enzyme, meaning that it uses inorganic phosphate as a cofactor to cleave nucleotide-nucleotide bonds,...

) but also can be complexes of multiple proteins, like the exosome complex

Exosome complex

The exosome complex is a multi-protein complex capable of degrading various types of RNA molecules...

 (in which four of the major exoribonuclease families are represented).

Major families

Family	Example members	Distribution	Catalytic activity
RNR	RNase R RNase R RNase R is an 3'-5' exoribonuclease closely related to RNase II, which has been shown to be involved in mRNA degradation in bacteria. RNase R has homologues in many other organisms. When a part of another larger protein has a domain that is very similar to RNase R, this is called an RNase R domain....	Most bacteria, all eukaryotes	3'-5' hydrolytic
	RNase II
	Rrp44
DEDD	RNase D RNase D RNase D is one of the seven exoribonucleases identified in E. coli. It is an 3'-5' exoribonuclease and which has been shown to be involved in the 3' processing of various stable RNA molecules ; It is known to add the 3' CCA sequence to tRNA in prokaryotic tRNA processing. RNase D has homologues...	Certain bacteria, all eukaryotes	3'-5' hydrolytic
	RNase T
	PM/Scl-100
	Oligoribonuclease
RBN	RNase BN	Certain bacteria	3'-5' hydrolytic
PDX	PNPase Polynucleotide phosphorylase Polynucleotide Phosphorylase is a bifunctional enzyme with a phosphorolytic 3' to 5' exoribonuclease activity and a 3'-terminal oligonucleotide polymerase activity. It is involved on mRNA processing and degradation in bacteria, plants, and in humans.In humans, the enzyme is encoded by the gene...	All kingdoms	3'-5' phosphorolytic
	PM/Scl-75
	RNase PH RNase PH RNase PH is an 3'-5' exoribonuclease and nucleotidyltransferase, present in archaea and bacteria, that is involved in tRNA processing. Contrary to hydrolytic enzymes, it is a phosphorolytic enzyme, meaning that it uses inorganic phosphate as a cofactor to cleave nucleotide-nucleotide bonds,...
RRP4	Rrp4	All eukaryotes, most archaea	3'-5' hydrolytic
5PX	Exoribonuclease I	All eukaryotes	5'-3' hydrolytic
	Exoribonuclease II

See also

• Endoribonuclease
  Endoribonuclease
  A Endoribonuclease is a ribonuclease endonuclease. It cleaves either single-stranded or double-stranded RNA, depending on the enzyme. Example includes both single proteins like RNase III, RNase A, RNase T1, and RNase H, and also complexes of proteins like RNase P and the RNA-induced silencing...
  
  
• Nuclease
  Nuclease
  A nuclease is an enzyme capable of cleaving the phosphodiester bonds between the nucleotide subunits of nucleic acids. Older publications may use terms such as "polynucleotidase" or "nucleodepolymerase"....
  
  
• Deoxyribonuclease
  Deoxyribonuclease
  A deoxyribonuclease is any enzyme that catalyzes the hydrolytic cleavage of phosphodiester linkages in the DNA backbone. Thus, deoxyribonucleases are one type of nuclease...
  
  
• Exodeoxyribonuclease
• Endodeoxyribonuclease
  Endodeoxyribonuclease
  Endodeoxyribonuclease is a endonuclease ribonuclease.Restriction enzymes are a type of endodeoxyribonuclease.More examples: Deoxyribonuclease I, Deoxyribonuclease II, Deoxyribonuclease IV, UvrABC endonuclease...