A DNA modifying enzyme or nuclease that digests nucleic acids starting from the 5' and/or 3' terminus and extending inward. The cleavage occurs by the hydrolytic action of the enzyme on the terminal phosphodiester bond. 










 The cell systems of eukaryotes and prokaryotes primarily have three types of exonucleases.





5' to 3' exonuclease (Xrn1): This a dependent mRNA decapping protein which removes the mRNA cap at the 5' end of the newly synthesized mRNA product. It is coded by the XRN1 gene. A 7-methylguanosine cap is usually synthesized at the 5' end of pre-mRNA which assists in ribosome binding. Additionally, a protein closely resembling it has been found to be involved in a variety of nuclear and cytoplasmic functions, including homologous recombination, meiosis, telomere maintenance, and microtubule assembly in yeast.





3' to 5' exonuclease: This exonuclease identifies the 3' end of the nucleic acid and digests the phosphodiester bonds inward. 





poly(A)-specific 3' to 5' exonuclease: A poly-A tail is synthesized in newly formed mRNA at the 3' terminus usually to add stability to the molecule. This exonuclease eats off the nucleotides upstream of the tail.








References:



Kaushansky, Kenneth. "Glossary of Molecular Biology Terminology." ASH Education Program Book 2000.1 (2000): 438-456.


Mukherjee D; et al. (2004). "Analysis of RNA Exonucleolytic Activities in Cellular Extracts". Springer protocols. 257: 193–211. doi:10.1385/1-59259-750-5:193. ISBN 1-59259-750-5. PMID 14770007.


Pamela A. Frischmeyer; et al. (2002). "An mRNA Surveillance Mechanism That Eliminates Transcripts Lacking Termination Codons". Science. 295 (5563): 2258–61. doi:10.1126/science.1067338. PMID 11910109.


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