MOLECULAR EVOLUTION OF THE HISTIDINE BIOSYNTHETIC-PATHWAY

The available sequences of genes encoding the enzymes associated with histidine biosynthesis suggest that this is an ancient metabolic pathw ay that was assembled prior to the diversification of the Bacteria, Ar chaea, and Eucarya. Paralogous duplications, gene elongation, and fusi on events involving different his genes have played a major role in sh aping this biosynthetic route. Evidence that the hisA and the hisF gen es and their homologues are the result of two successive duplication e vents that apparently took place before the separation of the three ce llular lineages is extended. These two successive gene duplication eve nts as well as the homology between the hisH genes and the sequences e ncoding the TrpG-type amidotransferases support the idea that during t he early stages of metabolic evolution at least parts of the histidine biosynthetic pathway were mediated by enzymes of broader substrate sp ecificities. Maximum likelihood trees calculated for the available seq uences of genes encoding these enzymes have been obtained. Their topol ogies support the possibility of an evolutionary proximity of archaeba cteria with low GC Gram-positive bacteria. This observation is consist ent with those detected by other workers using the sequences of heat-s hock proteins (HSP70), glutamine synthetases, glutamate dehydrogenases , and carbamoylphosphate synthetases.