Nucleoside-diphosphate kinase (NDP kinase), a key enzyme in nucleotide metabolism, is also known to be involved in growth and developmental control and tumor metastasis suppression. Interestingly, we find that coexpression of NDP kinase with Taz1, a Tar/EnvZ chimera, in the absence of its native signal, can activate a porin gene ompC-lacZ expression in Escherichia coli. Further studies show that NDP kinase can act as a protein kinase to phosphorylate histidine protein kinases such as EnvZ and CheA which are members of the HisAsp phosphorelay signal transduction systems in E. coli. Instead of ATP, the exclusive phosphodonor for histidine kinases, GTP can be utilized in vitro in the presence of NDP kinase to phosphorylate EnvZ and CheA, which then transfer the phosphoryl group to OmpR and CheY, the respective response regulators. The direct involvement of GTP for the phosphorylation of EnvZ through NDP kinase was further demonstrated by the use of a mutant EnvZ, which lost ability to be autophosphorylated with ATP. Phospho-OmpR thus formed can bind specifically to an ompF promoter sequence. These results suggest that NDP kinase may play a physiological role in signal transduction.Nucleoside-diphosphate kinase (NDP kinase) 1 is considered to play a key role in nucleotide metabolism to generate all nucleoside triphosphates from their corresponding nucleoside diphosphates using the ␥-phosphate from ATP or other (d)NTPs via a phosphoenzyme intermediate (1). The genes and structures of NDP kinases are highly conserved from Escherichia coli to human (43% identity) (2, 3), and NDP kinase is believed to be a housekeeping enzyme essential for DNA and RNA synthesis (4).Besides its role in nucleotide metabolism, NDP kinase is also involved in a number of cellular regulatory functions such as growth and developmental control and tumor metastasis suppression. Genetic analysis of NDP kinase from Myxococcus xanthus, a Gram-negative bacterium, has indicated that it may be essential for cell growth (5). The ndk gene from Schizosaccharomyces pombe was shown to regulate gene expression in sexual development in response to mating pheromone signaling (6). A null mutation of the awd gene, a Drosophila homologue of NDP kinase, results in abnormalities in larvae development (7). A cAMP receptor-stimulated NDP kinase activity was found in the cytoplasmic membrane of Dictyostelium discoideum which mediates the hormone action for the activation of G proteins (8). In addition, the genes of human and mouse NDP kinases/Nm23 have been shown to be involved in tumor metastasis suppression (9 -11). High metastatic potential of low nm23-expressing murine melanoma and human breast carcinoma cell lines was inhibited by transfection with nm23 cDNA. A human DNA-binding protein, PuF, identified as NDP kinase Nm23-H2, was shown to bind to the promoter region of c-myc in vitro and to activate c-myc transcription (12). Surprisingly, the ndk genes from E. coli, yeast, and S. pombe could be knocked out without affecting cell viability (6, 13, 14). However,...