The HIS6 gene freom Saccharomyces cerevisiae strain YNN282 is able to complement both the S. cerevisiae his6 and the Escherichia coli hisA mutations. The cloning and the nucleotide sequence indicated that this gene encodes a putative phosphoribosyl5amino1phosphoribosyl4imidazolecarboxiamide isomerase (5' ProFAR isomerase, EC 5.3.1.16) of 261 amino acids, with a molecular weight of 29 554. The HIS6 gene product shares a significant degree of sequence similarity with the prokaryotic HisA proteins and HisF proteins, and with the Cterminal domain of the S. cerevisiae HIS7 protein (homologous to HisF), indicating that the yeast HIS6 and HIS7 genes are paralogous. Moreover, the HIS6 gene is organized into two homologous modules half the size of the entire gene, typical of all the known prokaryotic hisA and hisF genes. The structure of the yeast HIS6 gene supports the twostep evolutionary model suggested by Fani et al. (J. Mol. Evol. 1994
38:489495) to explain the presentday hisA and hisF genes. According to this idea, the hisF gene originated freom the duplication of an ancestral hisA gene which, in turn, was the result of an earlier gene elongation event involving an ancestral module half the size of the extant gene. Results reported in this paper also suggest that these two successive paralogous gene duplications took probably place in the early steps of molecular evolution of the histidine pathway, well before the diversification of the three domains, and that this pathway was one of the metabolic activities of the last common ancestor. The molecular evolution of the yeast HIS6 and HIS7 genes is also discussed. (C) 1997 Elsevier Science B.V.
