Uracil-DNA glycosylase (EC 3.2.2.-) (UNG)  is a DNA repair enzyme that
excises uracil residues from DNA by cleaving the N-glycosylic bond. Uracil in
DNA can arise as a result of misincorportation of dUMP residues by DNA
polymerase or deamination of cytosine.
The sequence of uracil-DNA glycosylase is extremely well conserved  in
bacteria and eukaryotes as well as in herpes viruses. More distantly related
uracil-DNA glycosylases are also found in poxviruses .
In eukaryotic cells, UNG activity is found in both the nucleus and the
mitochondria. Human UNG1 protein is transported to both the mitochondria and
the nucleus . The N-terminal 77 amino acids of UNG1 seem to be required for
mitochondrial localization , but the presence of a mitochondrial transit
peptide has not been directly demonstrated.
As a signature for this type of enzyme, we selected the most N-terminal
conserved region. This region contains an aspartic acid residue which has been
proposed, based on X-ray structures [5,6] to act as a general base in the
In humans, two additional sequences of UNG have been reported [7,8].
These isozymes are not evolutionary related to other known UNG. One of them
is a glyceraldehyde 3-phosphate dehydrogenase  and the other related to
cyclins . Data available on three proteins proposed to be human uracil-DNA
glycosylases is discussed in .
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