The origin and evolution of eucaryal HIS7 genes: from metabolon to bifunctional proteins? Matteo Brilli, Renato Fani * Department of Animal Biology and Genetics, University of Florence, Via Romana 17-19, 50125 Firenze, Italy Received 18 February 2004; received in revised form 7 June 2004; accepted 17 June 2004 Received by G. Pesole Abstract The fifth step of histidine biosynthesis is catalysed by an imidazole glycerol-phosphate (IGP) synthase. In Archaea and Bacteria, the active form of IGP synthase is a stable 1:1 dimeric complex constituted by a glutamine amidotransferase (GAT) and a cyclase, the products of hisH and hisF. In Eucarya, the two activities are associated with a single bifunctional polypeptide encoded by HIS7 . In this work, we report a comparative analysis of the amino acid sequence of all the available HisH, HisF and HIS7 proteins, which allowed depicting a likely evolutionary pathway leading to the present-day bifunctional HIS7 genes. According to the model that we propose, the bifunctional HIS7 gene is the outcome of a gene fusion event between two independent ancestral cistrons encoding an amidotransferase and a cyclase, respectively. The phylogenetic distribution of the eucaryal HIS7 genes and the analysis of all the available prokaryotic counterparts (hisH and hisF) revealed the absence of such fusions in prokaryotes, suggesting that the fusion event very likely occurred in an early stage of eucaryal evolution and was fixed in the nucleated cells. The biological significance of this gene fusion is also discussed. D 2004 Elsevier B.V. All rights reserved. Keywords: Histidine biosynthesis; Gene fusion; Molecular evolution; his Genes evolution 1. Introduction The availability of nucleotide sequence of complete genomes from a constantly increasing number of (micro)- organisms belonging to the three cell domains, Archaea, Bacteria, and Eucarya is providing an enormous body of data concerning the structure and organisation of genes and genomes. Thus, it is now possible to disclose and clarify the molecular mechanisms responsible for their evolution and the shaping of metabolic pathways. In addition to gene duplication (Fani, 2004, and references therein), one of the major routes of gene evolution is the fusion of independent cistrons leading to bi- or multifunctional proteins (Brilli and Fani, 2004; Xie et al., 2003). Gene fusions provide a mechanism for the physical association of different catalytic domains or of catalytic and regulatory structures (Jensen, 1996). Fusions frequently involve genes coding for proteins that function in a concerted manner, such as enzyme catalysing sequential steps within a metabolic pathway (Yanai et al., 2002). Fusion of such catalytic centres likely promotes the channelling of intermediates that may be unstable and/or in low concentration (Jensen, 1996); this, in turn, requires that enzymes catalysing sequential reactions are colocalized within cell (Mathews, 1993) and may (transiently) interact to form complexes that are termed metabolons (Srere, 1987). The high fitness of gene fusions can also rely on the tight regulation of the expression of the fused domains. Even though gene fusion events have been described in many prokaryotes, they may have a special 0378-1119/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.gene.2004.06.033 Abbreviations: IGP, imidazole glycerol-phosphate; PRFAR, NV-(5V- phosphoribosyl)-formimino-5-aminoimidazol-4-carboxamide ribonucleo- tide; AICAR, 5V-(5-aminoimidazole-4-carboxamide) ribonucleotide; GAT, glutamine amidotransferase. * Corresponding author. Tel.: +39 55 2288244; fax: +39 55 2288250. E-mail address: r _ fani@dbag.unifi.it (R. Fani). Gene 339 (2004) 149 – 160 www.elsevier.com/locate/gene