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 Fe, Cu and Zn are essential cofactors for many enzymes. However, at high concentrations they are toxic. species belong to the class α-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 thymidylate kinase (MdTMPK), an essential enzyme in the synthesis of dTTP, thus being vital for cell growth. We show that Mg and Mn are the divalent metal ions required for catalysis and that Mn gives the highest catalytic efficiency. MdTMPK activity in the presence of Mg was strongly inhibited by the co-presence of Zn, Ni and Co. However, the addition of Cs caused >2-fold enhanced MdTMPK activity. For TMPK from and humans, the effects of Mg and Mn 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 .