Characterization and origin of bacterial arginine kinases

A search of over 1700 bacterial genomic sequences revealed eight species from Deinococcus-Thermus (Oceanithermus profundus) and Proteobacteria (Ahrensia sp., Nitratifractor salsuginis, Desulfobacterium autotrophicum, Desulfotalea psychrophila, Myxococcus xanthus, Moritella sp. and Sulfurovum sp.) possessing a complete AK sequence homologue. In addition, we searched another key protein that is homologous with that of the C-terminal domain of AK (mcsB). The mcsB is more widely distributed in about 150 species across at least nine bacterial genera. In agreement with the report by other authors, a phylogenetic tree of AK homologues shows that the eight species are separated into two clusters: cluster-A with AKs from ciliates Tetrahymena and Sterkiella and a porifera and the larger cluster-B, including most of the invertebrate AKs. We cloned and expressed the AK from Sulfurovum lithotrophicum in cluster-A and determined its enzymatic properties. Bacterial AKs were characterized as having the highest catalytic efficiency among known AKs, although there was a marked difference in kcat values for cluster-A and -B bacterial AKs. These observations suggest that bacterial AKs in cluster-B may be the prototype of invertebrate AKs. On the other hand, it appears that bacterial AKs in cluster-A diverged at an early stage of bacterial evolution after the appearance of AK, or introduced by horizontal gene transfer.