The evolution of the histidine biosynthetic genes in prokaryotes: a common ancestor for the hisA and hisF genes

The hisA and hisF genes belong to the histidine operon that has been extensively studied in the enterobacteria Escherichia coli and Salmonella typhimurium where the hisA gene codes for the phosphoribosyl-5-amino-1-phosphoribosyl-4-imidazolecarboxamide isomerase (EC 5.3.1.16) catalyzing the fourth step of the histidine biosynthetic pathway, and the hisF gene codes for a cyclase catalyzing the sixth reaction. Comparative analysis of nucleotide and predicted amino acid sequence of hisA and hisF genes in different microorganisms showed extensive sequence homology (43% considering similar amino acids), suggesting that the two genes arose from an ancestral gene by duplication and subsequent evolutionary divergence. A more detailed analysis, including mutual information, revealed an internal duplication both in hisA and hisF genes in each of the considered microorganisms. We propose that the hisA and hisF have originated from the duplication of a smaller ancestral gene corresponding to half the size of the actual genes followed by rapid evolutionary divergence. The involvement of gene elongation, gene duplication, and gene fusion in the evolution of the histidine biosynthetic genes is also discussed.