Glutamine: a major player in nitrogen catabolite repression in the yeast Dekkera bruxellensis

In the present work we studied the expression of genes from nitrogen central metabolism in the yeast Dekkera bruxellensis and under regulation by the Nitrogen Catabolite Repression mechanism (NCR). These analyses could shed some light on the biological mechanisms involved in the adaptation and survival of this yeast in the sugarcane fermentation process for ethanol production. Nitrogen sources (N-sources) in the form of ammonium, nitrate, glutamate or glutamine were investigated with or without the addition of methionine sulfoximine, which inhibits the activity of the enzyme glutamine synthetase and releases cells from NCR. The results showed that glutamine might act as an intracellular sensor for nitrogen availability in D. bruxellensis, by activating NCR. Gene expression analyses indicated the existence of two different GATA-dependent NCR pathways, identified as glutamine-dependent and glutamine-independent mechanisms. Moreover, nitrate is sensed as a non-preferential N-source and releases NCR to its higher level. After grouping genes according to their regulation pattern, we showed that genes for ammonium assimilation represent a regulon with almost constitutive expression, while permease encoding genes are mostly affected by the nitrogen sensor mechanism. On the other hand, nitrate assimilation genes constitute a regulon that is primarily subjected to induction by nitrate and, to a lesser extent, to a repressive mechanism by preferential N-sources. This observation explains our previous reports showing that nitrate is co-consumed with ammonium, a trait that enables D. bruxellensis cells to scavenge limiting N-sources in the industrial substrate and, therefore, to compete with Saccharomyces cerevisiae in this environment.