The role of themutTgene ofEscherichia coliin maintaining replication fidelity

Abstract

Spontaneous mutation levels are kept low in most organisms by a variety of error-reducing mechanisms, some of which ensure a high level of fidelity during DNA replication. The mutT gene of Escherichia coli is an important participant in avoiding such replication mistakes. An inactive mutT allele is a strong mutator with strict mutational specificity: only A.T-->C.G transversions are enhanced. The biological role of the MutT protein is thought to be the prevention of A.G mispairs during replication, specifically the mispair involving a template A and an oxidized form of guanine, 8-oxoguanine, which results when the oxidized form of dGTP, 8-oxodGTP, is available as a polymerase substrate. MutT is part of an elaborate defense system that protects against the mutagenic effects of oxidized guanine as a part of substrate dGTP and chromosomal DNA. The A.G mispairings prevented by MutT are not well-recognized and/or repaired by other fidelity mechanisms such as proofreading and mismatch repair, accounting in part for the high mutator activity of mutT. MutT is a nucleoside triphosphatase with a preference for the syn form of dGTP, hydrolyzing it to dGMP and pyrophosphate. 8-oxodGTP is hydrolyzed 10 times faster than dGTP, making it a likely biological substrate for MutT. MutT is assumed to hydrolyze 8-oxodGTP in the nucleotide pool before it can be misincorporated. While the broad role of MutT in error avoidance seems resolved, important details that are still unclear are pointed out in this review.

Cellular Defenses against Superoxide and Hydrogen Peroxide

“…Finally, MutT is an enzyme that hydrolyzes 8-oxodGTP to 8-oxodGMP. It has a strong impact upon mutagenesis (95), indicating both that 8-oxodGTP can be formed by the direct oxidation of the deoxynucleotide pool and that DNA polymerase III will misincorporate this analogue.…”

From Context-Dependence of Mutations to Molecular Mechanisms of Mutagenesis

“…A spontaneous mutation spectrum in the mutT deficient E.coli strain (186 mutations; lacI -d test system) is composed almost exclusively, of A•T→C•G transversions which is in general consistent with mutagenic properties of 8-oxoGTP [31]. Hotspot context analysis of these transversions [31] using the CLUSTERM program [22] and regression trees [32] revealed AA mutable sequence (the hotspot position is underlined) (Figure 3). Comparison of the mutTspectrum and A•T→C•T transversions in a spectrum of spontaneous mutations in the lacI gene (lacI -d test system) [33] did not reveal significant differences between them (probability that these two spectra are different [34] P(χ 2 ) = 0.69).…”

“…vivo and in vitro, depending whether guanine is oxidized in DNA or in the DNA precursor pools, respectively [30]. A spontaneous mutation spectrum in the mutT deficient E.coli strain (186 mutations; lacI -d test system) is composed almost exclusively, of A•T→C•G transversions which is in general consistent with mutagenic properties of 8-oxoGTP [31]. Hotspot context analysis of these transversions [31] using the CLUSTERM program [22] and regression trees [32] revealed AA mutable sequence (the hotspot position is underlined) (Figure 3).…”

Defects in DNA double-strand break repair re-sensitise antibiotic-resistant Escherichia coli to multiple bactericidal antibiotics

“…Antibiotic-induced oxidative stress can result in the formation of highly toxic and mutagenic oxidised nucleotide bases (eg. 8-oxo-dGTP) 46 , which must be cleared from the nucleotide pool by MutT before they are incorporated into the genome 27 . Although MTH1 (the MutT homologue in humans) holds promise as an anti-cancer therapeutic target 82 , disruption of mutT does not appear to be sufficiently important for bacterial cell survival or tolerance to be clinically useful.…”

“…A third pathway that is utilised under DNA damage conditions is nucleotide pool sanitation (NPS). The NPS pathway removes oxidized nucleotides from the resource pool and thus prevents insertion of aberrant bases during DNA synthesis 27 . In the absence of one particular NPS enzyme, MutT, insertion of the aberrant base 8-oxo-dGTP into the DNA triggers a form of maladaptive DNA repair that can kill bacterial cells 23 .…”