In a Pair of Paralogous Isozymes Catalyze the First Committed Step of Leucine Biosynthesis in Either the Mitochondria or the Cytosol.

Divergence of paralogous pairs, resulting from gene duplication, plays an important role in the evolution of specialized or novel gene functions. Analysis of selected duplicated pairs has elucidated some of the mechanisms underlying the functional diversification of (. ) paralogous genes. Similar studies of the orthologous pairs extant in pre-whole genome duplication yeast species, such as () remain to be addressed. The genome of , an aerobic yeast, includes gene pairs generated by sporadic duplications. The genome of this organism comprises the and paralogous pair, annotated as putative α-isopropylmalate synthases (α-IPMSs), considered to be the orthologs of the / paralogous genes. The enzymes encoded by the latter two genes are mitochondrially located, differing in their sensitivity to leucine allosteric inhibition resulting in Leu4-Leu4 and Leu4-Leu9 sensitive dimers and Leu9-Leu9 relatively resistant homodimers. Previous work has shown that, in a Δ mutant, expression is increased and assembly of Leu9-Leu9 leucine resistant homodimers results in loss of feedback regulation of leucine biosynthesis, leading to leucine accumulation and decreased growth rate. Here we report that: (i) harbors a sporadic gene duplication, comprising the , syntenic with and , and the non-syntenic , arising from a pre-WGD event. (ii) That both, and encode leucine sensitive α-IPMSs isozymes, located in the mitochondria (Leu4) and the cytosol (Leu4BIS), respectively. (iii) That both, or complement the Δ Δ leucine auxotrophic phenotype and revert the enhanced transcription observed in a Δ mutant. The Δ growth mutant phenotype is only fully complemented when transformed with the syntenic mitochondrial isoform. and underwent a different diversification pathways than that leading to /. could be considered as the functional ortholog of , since its encoded isozyme can complement both the Δ Δ leucine auxotrophy and the Δ complex phenotype.