Genome sequencing of Aspergillus glaucus 'CCHA' provides insights into salt-stress adaptation

Heterologous expression and characterization of salt-tolerant β-glucosidase from xerophilic Aspergillus chevalieri for hydrolysis of marine biomass

A salt-tolerant exo-β-1,3-glucosidase (BGL_MK86) was cloned from the xerophilic mold Aspergillus chevalieri MK86 and heterologously expressed in A. oryzae. Phylogenetic analysis suggests that BGL_MK86 belongs to glycoside hydrolase family 5 (aryl-phospho-β-D-glucosidase, BglC), and exhibits D-glucose tolerance. Recombinant BGL_MK86 (rBGL_MK86) exhibited 100-fold higher expression than native BGL_MK86. rBGL_MK86 was active over a wide range of NaCl concentrations [0%-18% (w/v)] and showed increased substrate affinity for p-nitrophenyl-β-D-glucopyranoside (pNPBG) and turnover number (kcat) in the presence of NaCl. The enzyme was stable over a broad pH range (5.5-9.5). The optimum reaction pH and temperature for hydrolysis of pNPBG were 5.5 and 45 °C, respectively. rBGL_MK86 acted on the β-1,3-linked glucose dimer laminaribiose, but not β-1,4-linked or β-1,6-linked glucose dimers (cellobiose or gentiobiose). It showed tenfold higher activity toward laminarin (a linear polymer of β-1,3 glucan) from Laminaria digitata than laminarin (β-1,3/β-1,6 glucan) from Eisenia bicyclis, likely due to its inability to act on β-1,6-linked glucose residues. The β-glucosidase retained hydrolytic activity toward crude laminarin preparations from marine biomass in moderately high salt concentrations. These properties indicate wide potential applications of this enzyme in saccharification of salt-bearing marine biomass.

Isolation and characterization of a salt-tolerant γ-glutamyl transpeptidase from xerophilic Aspergillus sydowii

Xerophilic Aspergillus molds isolated from halo-alkaliphilic and dry environments are attractive genetic resources for obtaining salt- and osmo-adaptive enzymes. A. sydowii MA0196 secreted the largest amount of γ-glutamyl transpeptidase (GGT) during solid-state fermentation at a low initial water activity (a w = 0.85). Gel filtration analysis revealed that the molecular mass of the purified native enzyme (MA0196 GGT) was 120 kDa. SDS-PAGE analysis showed that MA0196 GGT consists of two subunits with molecular masses of 56.4 and 33 kDa, indicating production from a proenzyme via autoproteolysis. Deglycosylation of the subunits by N-glycosidase F yielded 40.9 and 19.6 kDa species. MA0196 GGT retained transpeptidase and hydrolysis activities and their catalytic efficiency (k cat/K m) under high salt and low water activity. The enzyme displayed broad substrate specificity toward γ-glutamyl acceptors such as amino acids and the imidazole dipeptides, carnosine and anserine. Carnosine and L-glutamine were converted into γ-glutamyl-β-alanyl-L-histidine by MA0196 GGT with a 32.9% yield in the presence of 2% (v/v) dimethyl sulfoxide. Phylogenetic analysis indicated that MA0196 GGT forms a distinct lineage from A. oryzae and A. sojae GGTs. These excellent properties indicate that MA0196 GGT can be used in salted fermentation and for producing bioactive peptides.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03259-3.

Net Charges of the Ribosomal Proteins of the S10 and spc Clusters of Halophiles Are Inversely Related to the Degree of Halotolerance

Net positive charge(s) on ribosomal proteins (r-proteins) have been reported to influence the assembly and folding of ribosomes. A high percentage of r-proteins from extremely halophilic archaea are known to be acidic or even negatively charged. Those proteins that remain positively charged are typically far less positively charged. Here, the analysis is extended to non-archaeal halophilic bacteria, eukaryotes, and halotolerant archaea. The net charges (pH 7.4) of the r-proteins that comprise the S10-spc operon/cluster from individual microbial and eukaryotic genomes were estimated and intercompared. It was observed that, as a general rule, the net charges of individual proteins remained mostly basic as the salt tolerance of the bacterial strains increased from 5 to 15%. The most striking exceptions were the extremely halophilic bacterial strains, Salinibacter ruber SD01, Acetohalobium arabaticum DSM 5501 and Selenihalanaerobacter shriftii ATCC BAA-73, which are reported to require a minimum of 18% to 21% salt for their growth. All three strains have higher numbers of acidic S10-spc cluster r-proteins than what is seen in the moderate halophiles or the halotolerant strains. Of the individual proteins, only uL2 never became acidic. uS14 and uL16 also seldom became acidic. The net negative charges on several of the S10-spc cluster r-proteins are a feature generally shared by all extremely halophilic archaea and bacteria. The S10-spc cluster r-proteins of halophilic fungi and algae (eukaryotes) were exceptions: these were positively charged despite the halophilicity of the organisms. IMPORTANCE The net charges (at pH 7.4) of the ribosomal proteins (r-proteins) that comprise the S10-spc cluster show an inverse relationship with the halophilicity/halotolerance levels in both bacteria and archaea. In non-halophilic bacteria, the S10-spc cluster r-proteins are generally basic (positively charged), while the rest of the proteomes in these strains are generally acidic. On the other hand, the whole proteomes of the extremely halophilic strains are overall negatively charged, including the S10-spc cluster r-proteins. Given that the distribution of charged residues in the ribosome exit tunnel influences cotranslational folding, the contrasting charges observed in the S10-spc cluster r-proteins have potential implications for the rate of passage of these proteins through the ribosomal exit tunnel. Furthermore, the universal protein uL2, which lies in the oldest part of the ribosome, is always positively charged irrespective of the strain/organism it belongs to. This has implications for its role in the prebiotic context.