Heavy metal tolerance of Mesorhizobium delmotii thymidylate kinase

Metal ions play an important role in many metabolic processes in all living organisms. At low concentrations, heavy metals such as Fe2+, Cu2+ and Zn2+ are essential cofactors for many enzymes. However, at high concentrations they are toxic. Mesorhizobium species belong to the class alpha-proteobacteria and have high tolerance to soil acidity, salinity, temperature extremes, and metallicolous conditions. To identify factors responsible for this tolerance we have studied the effects of metal ions on Mesorhizobium delmotii thymidylate kinase (MdTMPK), an essential enzyme in the synthesis of dTTP, thus being vital for cell growth. We show that Mg2+ and Mn2+ are the divalent metal ions required for catalysis and that Mn2+ gives the highest catalytic efficiency. MdTMPK activity in the presence of Mg2+ was strongly inhibited by the co-presence of Zn2+, Ni2+ and Co2+. However, the addition of Cs+ caused >2-fold enhanced MdTMPK activity. For TMPK from Bacilus anthracis and humans, the effects of Mg2+ and Mn2+ were similar, whereas the effects of other divalent metal ions were different, and no stimulatory effect of Cs+ was observed. Together, our results demonstrate that MdTMPK and BaTMPK function well in the presence of high concentrations of heavy metal ions, introducing a potential contribution of these enzymes to the heavy metal tolerance of Mesorhizobium delmotii and Bacillus anthracis