Disruption of MRG19 results in altered nitrogen metabolic status and defective pseudohyphal development in Saccharomyces cerevisiae

Gene silencing of transgenes inserted in the Aspergillus nidulans alcM and/or alcS loci

While carrying out a systematic disruption of the genes of unknown function in the alc gene cluster from the filamentous fungus Aspergillus nidulans, we observed a strong diminution of the transcription of markers inserted in the alcS gene. This was found to be the case for the two markers tested, nadA (from A. nidulans) and pyrG (from A. fumigatus) involved in purine utilization and uracil/uridine biosynthetic pathway, respectively. The same phenomenon was also observed with insertion of the nadA gene in the alcM locus, another gene of the alc cluster. In the case of nadA, the level of expression was directly correlated to the ability of the corresponding strains to grow on adenine as a sole nitrogen source. The insertion of the pyrG marker into alcS complemented perfectly vegetative growth, but did not allow a proper sexual cycle. This suggests that the lowered pyrG expression is not sufficient to provide the intracellular concentration of pyrimidines required for the sexual cycle. Thus, due caution must be exercised when disrupting genes with pyrG, one of the most commonly employed markers, especially if the gene to be disrupted is involved or suspected to be involved in the sexual cycle.

The efficiency of mitochondrial electron transport chain is increased in the long-lived mrg19 Saccharomyces cerevisiae

Integrity of mitochondrial functionality is a key determinant of longevity in several organisms. In particular, reduced mitochondrial ROS (mtROS) production leading to decreased mtDNA damage is believed to be a crucial aspect of longevity. The generation of low mtROS was thought to be due to low mitochondrial oxygen consumption. However, recent studies have shown that higher mitochondrial oxygen consumption could still result in low mtROS and contribute to longevity. This increased mitochondrial efficiency (i.e. low mtROS generated despite high oxygen consumption) was explained as a result of mitochondrial biogenesis, which provides more entry points for the electrons to the electron transport chain (ETC), thereby resulting in low mtROS production. In this study, we provide evidence for the existence of an alternative pathway to explain the observed higher mitochondrial efficiency in the long-lived mrg19 mutant of Saccharomyces cerevisiae. Although we observe similar amounts of mitochondria in mrg19 and wild-type (wt) yeast, we find that mrg19 mitochondria have higher expression of ETC components per mitochondria in comparison with the wt. These findings demonstrate that more efficient mitochondria because of increased ETC per mitochondria can also produce less mtROS. Taken together, our findings provide evidence for an alternative explanation for the involvement of higher mitochondrial activity in prolonging lifespan. We anticipate that similar mechanisms might also exist in eukaryotes including human.

Mrg19 depletion increasesS. cerevisiaelifespan by augmenting ROS defence

Caloric restriction (CR) is the most compelling example of lifespan extension by external manipulation. Although the molecular mechanisms remain unknown, the theory of hormesis has been invoked to explain the life promoting effects of CR. Hormesis is defined as the beneficial effects of low intensity stressor on a cell or organism. Mrg19 is a putative transcription factor that regulates carbon and nitrogen metabolism in yeast. In this study, we have found that deletion of MRG19 gene causes metabolic shift in yeast cells, leading to higher intracellular reactive oxygen species, augmentation of scavenging enzymes and longer lifespan compared to wild-type cells. All these results together suggest that similar to CR, depletion of Mrg19 leads to a condition of mild stress which in turn enhances vitality.