1.9 Å structure of the signal receiver domain of the putative response regulator NarL from Mycobacterium tuberculosis

NarL from Mycobacterium tuberculosis is a putative nitrate response regulator that is involved in the regulation of anaerobic metabolism in this pathogen. The recombinant purified N-terminal signal receiver domain of NarL has been crystallized in space group C222₁, with unit-cell parameters a = 85.6, b = 90.0, c = 126.3 Å, and the structure was determined by molecular replacement to 1.9 Å resolution. Comparisons with related signal receiver domains show that the closest structural homologue is an uncharacterized protein from Staphylococcus aureus, whereas the nearest sequence homologue, NarL from Escherichia coli, displays larger differences in three-dimensional structure. The largest differences between the mycobacterial and E. coli NarL domains were found in the loop between β3 and α3 in the proximity of the phosphorylation site. The active site in response regulators is similar to that of members of the haloacid dehalogenase (HAD) family, which also form a phospho-aspartyl intermediate. In NarL, the aspartic acid that acts as catalytic acid/base in several HAD enzymes is replaced by an arginine residue, which is less likely to participate in steps involving proton abstraction. This substitution may slow down the breakdown of the phospho-aspartyl anhydride and allow signalling beyond the timescales defined by a catalytic reaction intermediate.