Identification of N(α)-acetyl-α-lysine as a probable thermolyte and its accumulation mechanism in Salinicoccus halodurans H3B36

Osmoregulation in freshwater anaerobic methane-oxidizing archaea under salt stress

Climate change-driven sea level rise threatens freshwater ecosystems and elicits salinity stress in microbiomes. Methane emissions in these systems are largely mitigated by methane-oxidizing microorganisms. Here, we characterized the physiological and metabolic response of freshwater methanotrophic archaea to salt stress. In our microcosm experiments, inhibition of methanotrophic archaea started at 1%. However, during gradual increase of salt up to 3% in a reactor over 12 weeks, the culture continued to oxidize methane. Using gene expression profiles and metabolomics, we identified a pathway for salt-stress response that produces the osmolyte of anaerobic methanotrophic archaea: N(ε)-acetyl-β-L-lysine. An extensive phylogenomic analysis on N(ε)-acetyl-β-L-lysine-producing enzymes revealed that they are widespread across both bacteria and archaea, indicating a potential horizontal gene transfer and a link to BORG extrachromosomal elements. Physicochemical analysis of bioreactor biomass further indicated the presence of sialic acids and the consumption of intracellular polyhydroxyalkanoates in anaerobic methanotrophs during salt stress.

Identification and characterization of a novel GNAT superfamily Nα -acetyltransferase from Salinicoccus halodurans H3B36

N^α -acetyl-α-lysine was found as a new type of compatible solutes that acted as an organic cytoprotectant in the strain of Salinicoccus halodurans H3B36. A novel lysine N^α -acetyltransferase gene (shkat), encoding an enzyme that catalysed the acetylation of lysine exclusively at α position, was identified from this moderate halophilic strain and expressed in Escherichia coli. Sequence analysis indicated ShKAT contained a highly conserved pyrophosphate-binding loop (Arg-Gly-Asn-Gly-Asn-Gly), which was a signature of the GNAT superfamily. ShKAT exclusively recognized free amino acids as substrate, including lysine and other basic amino acids. The enzyme showed a wide range of optimal pH value and was tolerant to high-alkali and high-salinity conditions. As a new member of the GNAT superfamily, the ShKAT was the first enzyme recognized free lysine as substrate. We believe this work gives an expanded perspective of the GNAT superfamily, and reveals great potential of the shkat gene to be applied in genetic engineering for resisting extreme conditions.

Development of the first gene expression system for Salinicoccus strains with potential application in bioremediation of hypersaline wastewaters

Salinicoccus salsiraiae IM408 (=CGMCC13032) is a novel halophilic bacterium that we isolated from the saline soil of Da Gang Oilfield. It tolerates 60 g/l sodium chloride and up to 123 g/l (1.5 M) sodium acetate and has shown a potential application in bioremediation of wastewater with high salt and high chemical oxygen demand (COD). Two plasmids, pS408-1 and pS408-2, were identified in S. salsiraiae IM408, and the sequences and copy numbers of the plasmids were determined. Based on these plasmids, two shuttle vectors containing a replicon for Escherichia coli, ampicillin, and chloramphenicol resistance genes, as well as the replicon from pS408-1 or pS408-2, were constructed and named as pTCS101 and pTCS201, respectively. A suitable host strain, named S. salsiraiae PE01, was also developed from the wild-type by plasmid elimination. Using the plasmid pTCS101 as an expression vector, L-lactate dehydrogenase from Staphylococcus aureus was expressed successfully in S. salsiraiae PE01. This is the first gene expression system for the Salinicoccus genus. It has provided the potential for expression of desired proteins or for establishment of desired pathways in Salinicoccus strains, which would make these halophiles more advantageous in future biotechnological applications.