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

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 .