DNA protection by the bacterial ferritin Dps via DNA charge transport

Abstract

Dps proteins, bacterial mini-ferritins that protect DNA from oxidative stress, are implicated in the survival and virulence of pathogenic bacteria. These proteins are thought to protect DNA by depleting ferrous iron and hydrogen peroxide, which can otherwise produce damaging hydroxyl radicals via Fenton chem. We seek to elucidate more specifically the mechanism of E. Coli Dps protection of DNA. DNA charge transport (CT), whereby charge is effectively conducted through the base pair π-stack, is proposed to be utilized within the cell in, for example, the long-range activation of redox-active transcription factors. Can the DNA-binding protein Dps similarly utilize DNA CT to protect the genome from a distance That is, must oxidizing equiv. diffuse specifically to the di-iron ferroxidase sites of Dps, or can Dps also become oxidized from a distance through DNA CT We employ an intercalating ruthenium photooxidant to generate oxidative DNA damage via the flash-quench technique. The injected electron hole localizes to guanine repeats, the sites of lowest potential in DNA. Because the lifetime of the guanine radical is long relative to the timescale of DNA CT, the guanine radical can interact with DNA-bound redox-active proteins. We find that Dps loaded with ferrous iron significantly attenuates the yield of oxidative DNA damage, in contrast to Apo-Dps and ferric iron-loaded Dps which lack reducing equiv. In this manner, ferrous iron-loaded Dps is selectively oxidized to fill guanine radical holes, thereby restoring the integrity of the DNA. Luminescence studies indicate no direct interaction between the ruthenium photooxidant and Dps, supporting the DNA-mediated oxidn. of ferrous iron-loaded Dps. Thus DNA CT may be a mechanism by which Dps efficiently protects the genome of pathogenic bacteria from a distance.

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