Differences in regulation of yeast gluconeogenesis revealed by Cat8p-independent activation of PCK1 and FBP1 genes in Kluyveromyces lactis.

Abstract

The yeast Kluyveromyces lactis is can utilise a wide range of non-fermentable carbon compounds as sole sources of carbon and energy, and differs from Saccharomyces cerevisiae in being able to carry out oxidative and fermentative metabolism simultaneously. In S. cerevisiae, growth on all non-fermentable carbon sources requires Cat8p, a transcriptional activator that controls the expression of gluconeogenic and glyoxylate cycle genes via CSREs (Carbon Source Responsive Elements). The down-regulation of Cat8p by fermentable carbon sources is the primary factor responsible for the tight repression of gluconeogenesis by glucose in S. cerevisiae. To analyse the regulation of gluconeogenesis in K. lactis, we have cloned and characterised the K. lactis homologue of CAT8 (KlCAT8). The gene was isolated by multicopy suppression of a fog2/klsnf1 mutation, indicating a similar epistatic relationship between KlSNF1 and KlCAT8 as in the case of the S. cerevisiae homologues. KlCAT8 encodes a protein of 1445 amino acids that is 40% identical to ScCat8p. The most highly conserved block is the putative Zn(II)2Cys6 DNA-binding domain, but additional conserved regions shared with members of the zinc-cluster family from Aspergillus define a subfamily of Cat8p-related proteins. KlCAT8 complements the growth defect of a Sccat8 mutant on non-fermentable carbon sources. In K. lactis, deletion of KlCAT8 severely impairs growth on ethanol, acetate and lactate, but not on glycerol. Derepression of enzymes of the glyoxylate cycle--malate synthase and particularly isocitrate lyase--was impaired in a Klcat8 mutant, whereas Northern analysis revealed that derepression of KlFBP1 and KlPCK1 does not require KlCat8p. Taken together, our results indicate that in K. lactis gluconeogenesis is not co-regulated with the glyoxylate cycle, and only the latter is controlled by KlCat8p.