A Structurally Divergent Class Ia Ribonucleotide Reductase from a Tick-Borne Pathogen

Ribonucleotide reductases (RNRs) generate 2'-deoxynucleotides for DNA biosynthesis, a reaction essential to all life. Class I RNRs have two subunits, α and β. α binds and reduces the substrate, whereas β oxidizes one of the cysteines in α to a C3'-H-bond-cleaving thiyl radical to begin the reaction. The α-Cys oxidant in β is variously a tyrosyl radical (Y^•) generated by a diiron or dimanganese cluster, a high-valent dimetal cluster [Mn(IV)/Fe(III) or Mn₂(IV/III)], or a dihydroxylphenylalanine (DOPA) radical that operates without need of a transition metal. The metal (in)dependence of the Cys oxidant in β correlates loosely with sequence-similarity groupings. We show here that Francisella hispaniensis (Fh) β, which lies within an uncharacterized sequence cluster that contains orthologs from multiple human pathogens, harbors a Fe₂(III/III)/Y^• cofactor, as in class Ia RNRs from eukaryotes and Escherichia coli. Fh β has several unusual structural features that may reflect adaptation to the bacterium's environment(s). In its apo form, an unwound helix everts a metal ligand toward solvent, and the radical-harboring Y points away from the diiron cluster. An additional aromatic residue (W194), conserved within the sequence cluster, is found close to the universally conserved W37, which is thought to mediate α-Cys oxidation in all class I enzymes. The Y^• in resting β is remarkably resistant to reduction by hydroxyurea but becomes 8000 times more sensitive when β is engaged in turnover with α. These structural and functional distinctions could be counter measures against host redox defenses that would target the pathogen's RNR and its cofactor.