Stress Reshapes the Physiological Response of Halophile Fungi to Salinity.

(1) Background: Mechanisms of cellular and molecular adaptation of fungi to salinity have been commonly drawn from halotolerant strains and few studies in basidiomycete fungi. These studies have been conducted in settings where cells are subjected to stress, either hypo- or hyperosmotic, which can be a confounding factor in describing physiological mechanisms related to salinity. (2) Methods: We have studied transcriptomic changes in Aspergillussydowii, a halophilic species, when growing in three different salinity conditions (No NaCl, 0.5 M, and 2.0 M NaCl). (3) Results: In this fungus, major physiological modifications occur under high salinity (2.0 M NaCl) and not when cultured under optimal conditions (0.5 M NaCl), suggesting that most of the mechanisms described for halophilic growth are a consequence of saline stress response and not an adaptation to saline conditions. Cell wall modifications occur exclusively at extreme salinity, with an increase in cell wall thickness and lamellar structure, which seem to involve a decrease in chitin content and an augmented content of alfa and beta-glucans. Additionally, three hydrophobin genes were differentially expressed under hypo- or hyperosmotic stress but not when the fungus grows optimally. Regarding compatible solutes, glycerol is the main compound accumulated in salt stress conditions, whereas trehalose is accumulated in the absence of salt. (4) Conclusions: Physiological responses to salinity vary greatly between optimal and high salt concentrations and are not a simple graded effect as the salt concentration increases. Our results highlight the influence of stress in reshaping the response of extremophiles to environmental challenges.

Genome mining for peptidases in heat-tolerant and mesophilic fungi and putative adaptations for thermostability.

BACKGROUND: Peptidases (EC 3.4) consist of a large group of hydrolytic enzymes that catalyze the hydrolysis of proteins accounting for approximately 65% of the total worldwide enzyme production. Peptidases from thermophilic fungi have adaptations to high temperature that makes them adequate for biotechnological application. In the present study, we profiled the genomes of heat-tolerant fungi and phylogenetically related mesophilic species for genes encoding for peptidases and their putative adaptations for thermostability. RESULTS: We generated an extensive catalogue of these enzymes ranging from 241 to 820 peptidase genes in the genomes of 23 fungi. Thermophilic species presented the smallest number of peptidases encoding genes in relation to mesophilic species, and the peptidases families with a greater number of genes were the most affected. We observed differences in peptidases in thermophilic species in comparison to mesophilic counterparts, at (i) the genome level: a great reduction in the number of peptidases encoding genes that harbored a higher number of copies; (ii) in the primary protein structure: shifts in proportion of single or groups of amino acids; and (iii) in the three-dimensional structure: reduction in the number of internal cavities. Similar results were reported for extremely thermophilic proteins, but here we show for the first time that several changes also occurred on the moderate thermophilic enzymes of fungi. In regards to the amino acids composition, peptidases from thermophilic species in relation to the mesophilic ones, contained a larger proportion of Ala, Glu, Gly, Pro, Arg and Val residues and a lower number of Cys, His, Ile, Lys, Met, Asn, Gln, Ser, Thr and Trp residues (P < 0.05). Moreover, we observed an increase in the proportion of hydrophobic and charged amino acids and a decrease in polar amino acids. CONCLUSIONS: Although thermophilic fungi present less genes encoding for peptidases, these have adaptations that could play a role in thermal resistance from genome to protein structure level.