Nucleoid structure and dynamics influence natural chromosomal transformation in <i>Bacillus subtilis</i>: the role of EbfC

Natural chromosomal transformation (NCT) in Bacillus subtilis requires RecA and its accessory proteins including RecX and RecD2. Inactivation of the nucleoid-associated protein (NAP) Hbsu, Rok, or LrpC results in a dual effect: it enhances NCT but exacerbates the NCT defect of ΔrecX cells. Purified EbfC exhibits characteristic features of NAPs: it binds both single- and double-stranded DNA, protects them from degradation, and forms higher-order protein-DNA complexes via its DNA bridging activity. NCT is reduced upon EbfC inactivation but enhanced by ebfC overexpression. hbs55, Δrok, or ΔlrpC mutations suppress the NCT defect of ΔebfC, yet synergistically increase NCT upon ebfC overexpression. The NCT defect in ΔrecD2 cells is worsened by ebfC overexpression or by hbs55, Δrok, or ΔlrpC mutations. The nucleoid was more compacted in ΔebfC cells, an effect counteracted by hbs55, Δrok, or ΔlrpC mutations. EbfC contributes to DNA repair, and ebfC is epistatic to hbs or lrpC in response to DNA damage. We propose that chromosome folding, modulated by NAPs, plays a critical role in NCT and DNA repair. In this context, EbfC, by regulating nucleoid dynamics as a NAP, opposes the functions of Hbsu, Rok, and LrpC in NCT, while their interconnected roles contribute to DNA repair.