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 e xtensively studied in the enterobacteria Escherichia coli and Salmonel la typhimurium where the hisA gene codes for the phosphoribosyl-5-amin o-1 -phosphoribosyl-4-imidazolecarboxamide isomerase (EC 5.3.1.16) cat alyzing the fourth step of the histidine biosynthetic pathway, and the hisF gene codes for a cyclase catalyzing the sixth reaction. Comparat ive analysis of nucleotide and predicted amino acid sequence of hisA a nd hisF genes in different microorganisms showed extensive sequence ho mology (43% considering similar amino acids), suggesting that the two genes arose from an ancestral gene by duplication and subsequent evolu tionary divergence. A more detailed analysis, including mutual informa tion, revealed an internal duplication both in hisA and hisF genes in each of the considered microorganisms. We propose that the hisA and hi sF have originated from the duplication of a smaller ancestral gene co rresponding to half the size of the actual genes followed by rapid evo lutionary divergence. The involvement of gene elongation, gene duplica tion, and gene fusion in the evolution of the histidine biosynthetic g enes is also discussed.