Transcriptional upregulation of four genes of the lysine biosynthetic pathway by homocitrate accumulation in Penicillium chrysogenum: homocitrate as a sensor of lysine-pathway distress

Abstract

The lysine biosynthetic pathway has to supply large amounts ofα-aminoadipic acid for penicillin biosynthesis inPenicillium chrysogenum. In this study, we have characterized theP. chrysogenumL2 mutant, a lysine auxotroph that shows highly increased expression of several lysine biosynthesis genes (lys1,lys2,lys3,lys7). The L2 mutant was found to be deficient in homoaconitase activity since it was complemented by theAspergillus nidulans lysFgene. We have cloned a gene (namedlys3) that complements the L2 mutation by transformation with aP. chrysogenumgenomic library, constructed in an autonomous replicating plasmid. Thelys3-encoded protein showed high identity to homoaconitases. In addition, we cloned the mutantlys3allele from the L2 strain that showed a G1534to A1534point mutation resulting in a Gly495to Asp495substitution. This mutation is located in a highly conserved region adjacent to two of the three cysteine residues that act as ligands to bind the iron–sulfur cluster required for homoaconitase activity. The L2 mutant accumulates homocitrate. Deletion of thelys1gene (homocitrate synthase) in the L2 strain prevented homocitrate accumulation and reverted expression levels of the four lysine biosynthesis genes tested to those of the parental prototrophic strain. Homocitrate accumulation seems to act as a sensor of lysine-pathway distress, triggering overexpression of four of the lysine biosynthesis genes.