Plasmid expression of Deinococcus radiodurans RecA confers UV-A protection to Escherichia coli with an inverse protein dose dependence, which does not exceed conspecific RecA protection

Low level expression in Escherichia coli of the RecA protein from the radiation resistant bacterium Deinococcus radiodurans protects a RecA deficient strain of E. coli from UV-A irradiation by up to ∼160% over basal UV-A resistance. The protection effect is inverse protein dose dependent: increasing the expression level of the D. radiodurans RecA (DrRecA) protein decreases the protection factor. This inverse protein dose dependence effect helps resolve previously conflicting reports of whether DrRecA expression is protective or toxic for E. coli. In contrast to the D. radiodurans protein effect, conspecific plasmid expression of E. coli RecA protein in RecA deficient E. coli is consistently protective over several protein expression levels, as well as consistently more protective to higher levels of UV-A exposure than that provided by the D. radiodurans protein. The results indicate that plasmid expression of D. radiodurans RecA can modestly enhance the UV resistance of living E. coli, but that the heterospecific protein shifts from protective to toxic as expression is increased.

Deinococcus lineage and Rad52 family-related protein DR0041 is involved in DNA protection and compaction

DR0041 ORF encodes an uncharacterized Deinococcus lineage protein. We earlier reported presence of DR0041 protein in DNA repair complexes of Ssb and RecA in Deinococcus radiodurans. Here, we systematically examined the role of DR0041 in DNA metabolism using various experimental methodologies including electrophoretic mobility assays, nuclease assays, strand exchange assays and transmission electron microscopy. Interaction between DR0041 and the C-terminal acidic tail of Ssb was assessed through co-expression and in vivo cross-linking studies. A knockout mutant was constructed to understand importance of DR0041 ORF for various physiological processes. Results highlight binding of DR0041 protein to single-stranded and double-stranded DNA, interaction with Ssb-coated single-stranded DNA without interference with RecA-mediated strand exchange, protection of DNA from exonucleases, and compaction of high molecular weight DNA molecules into tightly condensed forms. Bridging and compaction of sheared DNA by DR0041 protein might have implications in the preservation of damaged DNA templates to maintain genome integrity upon exposure to gamma irradiation. Our results suggest that DR0041 protein is dispensable for growth under standard growth conditions and following gamma irradiation but contributes to protection of DNA during transformation. We discuss the role of DR0041 protein from the perspective of protection of broken DNA templates and functional redundancy.