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 the
endonuclease, which cleaves
phosphodiester bonds in the middle (endo) of a polynucleotide chain.
Eukaryotes and
prokaryotes have three types of exonucleases involved in the normal turnover of
mRNA: 5’ to 3’ exonuclease, which is a dependent
decapping protein; 3’ to 5’ exonuclease, an independent protein; and poly(A)-specific 3’ to 5’ exonuclease.<ref name">
