The serine/threonine phosphatase DhSIT4 modulates cell cycle, salt tolerance and cell wall integrity in halo tolerant yeast Debaryomyces hansenii

Efficient PCR-based gene targeting in isolates of the nonconventional yeast Debaryomyces hansenii

Debaryomyces hansenii is a yeast with considerable biotechnological potential as an osmotolerant, stress-tolerant oleaginous microbe. However, targeted genome modification tools are limited and require a strain with auxotrophic markers. Gene targeting by homologous recombination has been reported to be inefficient, but here we describe a set of reagents and a method that allows gene targeting at high efficiency in wild-type isolates. It uses a simple polymerase chain reaction (PCR)-based amplification that extends a completely heterologous selectable marker with 50 bp flanks identical to the target site in the genome. Transformants integrate the PCR product through homologous recombination at high frequency (>75%). We illustrate the potential of this method by disrupting genes at high efficiency and by expressing a heterologous protein from a safe chromosomal harbour site. These methods should stimulate and facilitate further analysis of D. hansenii strains and open the way to engineer strains for biotechnology.

Contribution of the mitogen-activated protein kinase Hog1 to the halotolerance of the marine yeast Debaryomyces hansenii

Halotolerant species are adapted to dealing continually with hyperosmotic environments, having evolved strategies that are uncommon in other organisms. The HOG pathway is the master system that regulates the cellular adaptation under these conditions; nevertheless, apart from the importance of Debaryomyces hansenii as an organism representative of the halotolerant class, its HOG1 pathway has been poorly studied, due to the difficulty of applying conventional recombinant DNA technology. Here we describe for the first time the phenotypic characterisation of a null HOG1 mutant of D. hansenii. Dhhog1Δ strain was found moderately resistant to 1 M NaCl and sensitive to higher concentrations. Under hyperosmotic shock, DhHog1 fully upregulated transcription of DhSTL1 and partially upregulated that of DhGPD1. High osmotic stress lead to long-term inner glycerol accumulation that was partially dependent on DhHog1. These observations indicated that the HOG pathway is required for survival under high external osmolarity but dispensable under low and mid-osmotic conditions. It was also found that DhHog1 can regulate response to alkali stress during hyperosmotic conditions and that it plays a role in oxidative and endoplasmic reticulum stress. Taken together, these results provide new insight into the contribution of this MAPK in halotolerance of this yeast.

PP2A-Like Protein Phosphatase (Sit4) Regulatory Subunits, Sap155 and Sap190, Regulate Candida albicans' Cell Growth, Morphogenesis, and Virulence

PP2A-like phosphatases share high homology with PP2A enzymes and are composed of a catalytic subunit and a regulatory subunit. In Candida albicans, the PP2A-like catalytic subunit SIT4 regulates cell growth, morphogenesis, and virulence. However, the functions of its regulatory subunits remain unclear. Here, by homology analysis and co-IP experiments, we identified two regulatory subunits of SIT4 in C. albicans, SAP155 (orf19.642) and SAP190 (orf19.5160). We constructed sit4Δ/Δ, sap155Δ/Δ, sap190Δ/Δ, and sap155Δ/Δ sap190Δ/Δ mutants and found that deleting SAP155 had no apparent phenotypic consequence, while deleting SAP190 caused slow growth, hypersensitivity to cell wall stress, abnormal morphogenesis in response to serum or genotoxic stress (HU and MMS), less damage to macrophages, and attenuated virulence in mice. However, deleting both SAP155 and SAP190 caused significantly stronger defects, which was similar to deleting SIT4. Together, our results suggest that SAP190 is required for the function of SIT4 and that SAP155 can partially compensate for the loss of SAP190 in C. albicans. Given the vital role of these regulatory subunits of SIT4 in C. albicans physiology and virulence, they could serve as potential antifungal targets.

Overlapping responses between salt and oxidative stress in Debaryomyces hansenii

Debaryomyces hansenii is a halotolerant yeast of importance in basic and applied research. Previous reports hinted about possible links between saline and oxidative stress responses in this yeast. The aim of this work was to study that hypothesis at different molecular levels, investigating after oxidative and saline stress: (i) transcription of seven genes related to oxidative and/or saline responses, (ii) activity of two main anti-oxidative enzymes, (iii) existence of common metabolic intermediates, and (iv) generation of damages to biomolecules as lipids and proteins. Our results showed how expression of genes related to oxidative stress was induced by exposure to NaCl and KCl, and, vice versa, transcription of some genes related to osmotic/salt stress responses was regulated by H₂O₂. Moreover, and contrary to S. cerevisiae, in D. hansenii HOG1 and MSN2 genes were modulated by stress at their transcriptional level. At the enzymatic level, saline stress also induced antioxidative enzymatic defenses as catalase and glutathione reductase. Furthermore, we demonstrated that both stresses are connected by the generation of intracellular ROS, and that hydrogen peroxide can affect the accumulation of in-cell sodium. On the other hand, no significant alterations in lipid oxidation or total glutathione content were observed upon exposure to both stresses tested. The results described in this work could help to understand the responses to both stressors, and to improve the biotechnological potential of D. hansenni.

Biological features and regulatory mechanisms of salt tolerance in plants

Halophytes play a vital role in saline agriculture because these plants are necessary to increase the food supply to meet the demands of the growing world population. In addition, the transfer of salt-resistance genes from halophytes using genetic technologies has the potential to increase the salt tolerance of xerophytes. Characterization of some particularly promising halophyte model organisms has revealed the important new insights into the salt tolerance mechanisms used by plants. Numerous advances using these model systems have improved our understanding of salt tolerance regulation and salt tolerance-associated changes in gene expression, and these mechanisms have important implications for saline agriculture. Recent findings provide a basis for future studies of salt tolerance in plants, as well as the development of improved strategies for saline agriculture to increase yields of food, feed, and fuel crops.