A dual signalling pathway for the hypoxic expression of lipid genes, dependent on the glucose sensor Rag4, is revealed by the analysis of the KlMGA2 gene in Kluyveromyces lactis In the respiratory yeast Kluyveromyces lactis, little is known about the factors regulating the metabolic response to oxygen shortage. After searching for homologues of characterized Saccharomyces cerevisiae regulators of the hypoxic response, we identified a gene that we named KlMGA2, which is homologous to MGA2. The deletion of KlMGA2 strongly reduced both the fermentative and respiratory growth rate and altered fatty acid composition and the unsaturation index of membranes. The reciprocal heterologous expression of MGA2 and KlMGA2 in the corresponding deletion mutant strains suggested that Mga2 and KlMga2 are functional homologues. KlMGA2 transcription was induced by hypoxia and the glucose sensor Rag4 mediated the hypoxic induction of KlMGA2. Transcription of lipid biosynthetic genes KlOLE1, KlERG1, KlFAS1 and KlATF1 was induced by hypoxia and was dependent on KlMga2, except for KlOLE1. Rag4 was required for hypoxic induction of transcription for both KlMga2-dependent (KlERG1) and KlMga2-independent (KlOLE1) structural genes.

Kluyveromyces lactis is a yeast which cannot grow under strict anaerobiosis. To date, no factors responsible for oxygen sensing and oxygen-dependent regulation of metabolism have been identified. In this paper we present the identification of the glucose sensor Rag4 as a factor essential for oxygen-dependent regulation of the fermentative pathway.

In Pseudomonas fluorescens ST, the expression of the pathway for the degradation of the toxic compound styrene is induced by the substrate (i.e., styrene) and repressed by alternative carbon sources. This regulation relies on a two-component system consisting of the sensor kinase StyS and the response regulator StyR. StyS belongs to the group of hybrid “double sensor kinases”, a unique family of sensor kinases having two ATP catalytic domains, each one preceded by a PAS domain. Here, the styrene-based phosphorylation of StyS is reported. Styrene is specifically sensed by the StyS N-terminal PAS sensor domain, enhancing the StyS phosphorylation level

In the yeast Kluyveromyces lactis, the pyruvate decarboxylase gene KlPDC1 is strongly regulated at the transcription level by different environmental factors. Sugars and hypoxia act as inducers of transcription, while ethanol acts as a repressor. Their effects are mediated by gene products, some of which have been characterized. KlPDC1 transcription is also strongly repressed by its product – KlPdc1 – through a mechanism called autoregulation. We performed a genetic screen that allowed us to select and identify the regulatory gene RAG3 as a major factor in the transcriptional activity of the KlPDC1 promoter in the absence of the KlPdc1 protein, i.e. in the autoregulatory mechanism. We also showed that the two proteins Rag3 and KlPdc1 interact, co-localize in the cell and that KlPdc1 may control Rag3 nuclear localization.