Increased base change mutations at G:C pairs in Escherichia coli deficient in endonuclease III and VIII

DNA-mediated signaling by proteins with 4Fe-4S clusters is necessary for genomic integrity

Iron-sulfur clusters have increasingly been found to be associated with enzymes involved in DNA processing. Here we describe a role for these redox clusters in DNA-mediated charge-transport signaling in E. coli between DNA repair proteins from distinct pathways. DNA-modified electrochemistry shows that the 4Fe-4S cluster of DNA-bound DinG, an ATP-dependent helicase that repairs R-loops, is redox-active at cellular potentials and ATP hydrolysis increases DNA-mediated redox signaling. Atomic force microscopy experiments demonstrate that DinG and Endonuclease III (EndoIII), a base excision repair enzyme, cooperate at long-range using DNA charge transport to redistribute to regions of DNA damage. Genetics experiments, moreover, reveal that this DNA-mediated signaling among proteins also occurs within the cell and, remarkably, is required for cellular viability under conditions of stress. Silencing the gene encoding EndoIII in a strain of E. coli where repair by DinG is essential results in a significant growth defect that is rescued by complementation with EndoIII but not with an EndoIII mutant that is enzymatically active but unable to carry out DNA charge transport. This work thus elucidates a fundamental mechanism to coordinate the activities of DNA repair enzymes across the genome.

Substitution of Ala for Tyr567 in RB69 DNA polymerase allows dAMP and dGMP to be inserted opposite Guanidinohydantoin

Continuous oxidative damage inflicted on DNA produces 7,8-dihydro-8-oxoguanine (8-oxoG), a commonly occurring lesion that can potentially cause cancer by producing G → T transversions during DNA replication. Mild oxidation of 8-oxoG leads to the formation of hydantoins, specifically guanidinohydantoin (Gh) and spiroiminodihydantoin (Sp), which are 100% mutagenic because they encode almost exclusively the insertion of dAMP and dGMP (encoding G → T and G → C transversions, respectively). The wild-type (wt) pol α family DNA polymerase from bacteriophage RB69 (RB69pol) inserts dAMP and dGMP with low efficiency when situated opposite Gh. In contrast, the RB69pol Y567A mutant inserts both of these dNMPs opposite Gh with >100-fold higher efficiency than wt. We now report the crystal structure of the "closed" preinsertion complex for the Y567A mutant with dATP opposite a templating Gh (R-configuration) in a 13/18mer primer-template (P/T) at 2.0 Å resolution. The structure data reveal that the Y to A substitution provides the nascent base pair binding pocket (NBP) with the flexibility to accommodate Gh by allowing G568 to move in the major-to-minor groove direction of the P/T. Thus, Gh is rejected as a templating base by wt RB69pol because G568 is inflexible, preventing Gh from pairing with the incoming dATP or dGTP base.

DNA repair glycosylases with a [4Fe-4S] cluster: a redox cofactor for DNA-mediated charge transport?

The [4Fe-4S] cluster is ubiquitous to a class of base excision repair enzymes in organisms ranging from bacteria to man and was first considered as a structural element, owing to its redox stability under physiological conditions. When studied bound to DNA, two of these repair proteins (MutY and Endonuclease III from Escherichia coli) display DNA-dependent reversible electron transfer with characteristics typical of high potential iron proteins. These results have inspired a reexamination of the role of the [4Fe-4S] cluster in this class of enzymes. Might the [4Fe-4S] cluster be used as a redox cofactor to search for damaged sites using DNA-mediated charge transport, a process well known to be highly sensitive to lesions and mismatched bases? Described here are experiments demonstrating the utility of DNA-mediated charge transport in characterizing these DNA-binding metalloproteins, as well as efforts to elucidate this new function for DNA as an electronic signaling medium among the proteins.