Interestingly, all NMP kinases, except for UMPK, converted the NMP substrates to the respective NTPs with an NDP as intermediate (Figure ). Based on these results we tested the ability of the NMPK kinases to accept NDPs as substrates with ATP as phosphate donor. Indeed, TmAMPK, TmGMPK and Tm(d)CMPK were able to catalyze NTP formation with all tested natural (deoxy)NDPs (Figure ). TmTMPK and TmUMPK catalyzed (d)ATP formation, and TmTMPK additionally converted TDP to TTP. Thus, T. maritima NDPK deficiency might be compensated by TmAMPK, TmGMPK and Tm(d)CMPK. NDPK activity of NMPKs has already been described before for diverse AMPKs , and human UMP-CMPK . Human NMPKs were described to show real NDPK activity (NDP + ATP ↔ NTP + ADP) . In contrast, NDP kinase activity of bacterial AMPKs was explained by the commonly known reversibility of the NMPK reaction (N MP + N TP ↔ N DP + N DP) .