Diversification of Transcriptional Regulation Determines Subfunctionalization of Paralogous Branched Chain Aminotransferases in the Yeast Saccharomyces cerevisiae

harbors and paralogous genes that encode branched chain aminotransferases and have opposed expression profiles and physiological roles . Accordingly, in primary nitrogen sources such as glutamine, expression is induced, supporting Bat1-dependent valine-isoleucine-leucine (VIL) biosynthesis, while expression is repressed. Conversely, in the presence of VIL as the sole nitrogen source, expression is hindered while that of is activated, resulting in Bat2-dependent VIL catabolism. The presented results confirm that expression is determined by transcriptional activation through the action of the Leu3-α-isopropylmalate (α-IPM) active isoform, and uncovers the existence of a novel α-IPM biosynthetic pathway operating in a Δ mutant grown on VIL, through Bat2-Leu2-Leu1 consecutive action. The classic α-IPM biosynthetic route operates in glutamine through the action of the leucine-sensitive α-IPM synthases. The presented results also show that repression in glutamine can be alleviated in a Δ mutant or through Gcn4-dependent transcriptional activation. Thus, when is grown on glutamine, VIL biosynthesis is predominant and is preferentially achieved through ; while on VIL as the sole nitrogen source, catabolism prevails and is mainly afforded by .