Biology Reference
In-Depth Information
subsequently cloned
2
and the protein further characterized in a number of
laboratories.
3-6
E. coli mutM
mutants were identified in Jeffrey Miller's labo-
ratory as mutators that gave rise to G
T transversions.
7
When the MutM
protein was subsequently purified, it was found to be identical to Fpg.
8
Fol-
lowing these initial findings, there were a number of biochemical studies
showing that 8-oxoguanine (8-oxoG) was also a substrate for Fpg and that
Fpg preferred 8-oxoG over methylFapyG.
9,10
Because of this substrate prefer-
ence and because guanine is the most readily oxidized DNA base, the conclu-
sion was drawn that 8-oxoguanine was the biologically relevant substrate for
Fpg. These studies led to the formulation of the GO model for 8-oxoG repair,
11
which proposed that when guanine is oxidized to 8-oxoguanine, it is removed
by Fpg. If 8-oxoG is not removed prior to replication, A is often inserted
opposite the 8-oxoG by DNA polymerases.
12-15
If this occurs, the A can be
removed by another glycosylase called MutY.
16
The GO model also included
MutT, which removes 8-oxoguanine nucleoside triphosphates from the nucle-
otide pool by hydrolyzing them to 8-oxodGMP.
17
Taken together, these data
supported the idea that 8-oxoguanine is a biologically important, potentially
mutagenic oxidative DNA lesion. However, recent studies have shown that
unmethylated FapyG is also a good substrate for Fpg
18,19
and like 8-oxoG,
A can also be incorporated opposite FapyG
20,21
and the incorporated A can be
removed by MutY.
22
FapyG, which is formed from the same adduct radical as
8-oxoG,
23
appears to be responsible for a substantial number of mutations
originally attributed to 8-oxoG and thus is also a biologically relevant
substrate.
24
E. coli nei
(endonuclease VIII) was originally discovered in the Wallace
laboratory as an activity that recognizes oxidized pyrimidines.
25,26
The gene was
cloned and the protein sequence was shown to be very similar to that of Fpg.
27
nei
mutants had little or no phenotype, but, when coupled with an
nth
muta-
tion, they were mutators leading to C
!
T transitions.
27
The
nth
gene encodes
endonuclease III, which also recognizes oxidized pyrimidines with a substrate
specificity that substantially overlaps that of Nei (for reviews see Refs.
28,29
).
It was not until the twenty-first century and the sequencing of the human
genome that
in silico
analysis allowed the Wallace, Mitra, and Seeberg labora-
tories to identify, clone, and characterize three Fpg/Nei homologs in mamma-
lian cells, the so-called Neil1 (nei-like), Neil2, and Neil3 proteins.
30-34
Mouse Neil1 and Neil3 were also found in mice nullizygous for
nth
.
35
The substrate specificities of human NEIL1 and NEIL2 have been well
characterized.
30-34,36-40
In addition, NEIL1 forms specific interactions with a
number of replication proteins and is cell cycle-regulated.
41-44
Thus, it has
been proposed that NEIL1 acts as a cow catcher ahead of the replication fork,
eliminating potentially mutagenic lesions.
42-44
NEIL2 prefers lesions in single-
stranded DNA over those in duplex DNA and interacts with a number of
!
