An Extremely Halophilic Proteobacterium Combines a Highly Acidic Proteome with a Low Cytoplasmic Potassium Content

Abstract

Background:The molecular cause and adaptive advantage of proteome halophilicity and acidity in haloarchaea accumulating KCl for osmoprotection are unresolved. Results: Halorhodospira halophila has an acidic proteome and accumulates molar concentrations of KCl but only when grown in hypersaline medium. Conclusion: KCl accumulation occurs in Proteobacteria and does not necessitate proteome halophilicity. Significance: Proteome acidity is needed for protein surface hydration; obligate proteome halophilicity results from constructive neutral evolution.Halophilic archaea accumulate molar concentrations of KCl in their cytoplasm as an osmoprotectant and have evolved highly acidic proteomes that function only at high salinity. We examined osmoprotection in the photosynthetic Proteobacteria Halorhodospira halophila and Halorhodospira halochloris. Genome sequencing and isoelectric focusing gel electrophoresis showed that the proteome of H. halophila is acidic. In line with this finding, H. halophila accumulated molar concentrations of KCl when grown in high salt medium as detected by x-ray microanalysis and plasma emission spectrometry. This result extends the taxonomic range of organisms using KCl as a main osmoprotectant to the Proteobacteria. The closely related organism H. halochloris does not exhibit an acidic proteome, matching its inability to accumulate K ؉ . This observation indicates recent evolutionary changes in the osmoprotection strategy of these organisms. Approximately 97% of all water on earth is present in saline oceans, saline lakes, inland seas, and saline groundwater (1), and roughly one-quarter of the land on earth is underlain by salt deposits (2). Thus, saline and hypersaline environments are highly abundant and of great ecological significance. In addition, salinity is a major determinant for microbial community composition (3). Therefore, halophilic adaptations are of general biological interest. Most extreme halophiles are members of the Halobacteria (Archaea), and particularly Halobacterium salinarum has been studied extensively (4). Extreme halophilicity in bacteria is less well studied but has been described for the chemotroph Salinibacter ruber and the photosynthetic purple bacterium Halorhodospira halophila (5, 6). A key factor in the halophilic adaptations of H. salinarum and S. ruber is that they accumulate up to 5 M KCl in their cytoplasm (7)(8)(9)(10)(11). This osmoprotection strategy results in unidentified protein-solvent interactions that drive a proteome-wide adaptation in which all proteins have an acidic isoelectric point due to an excess of Glu and Asp residues (4, 12-16). The taxonomic distribution of the reported use of KCl as a major osmoprotectant in extreme halophiles is quite limited: it has been reported only in Halobacteria, in S. ruber (Bacteroidetes), and, to a somewhat lesser extent, in the Halanaerobiales (Firmicutes) (8, 9).Many enzymes from extreme halophiles with acidic proteomes require the presence of at least 1 M salt to be stable and active (12)(13)(14). Stabi...

Common and distinguishing features of the bacterial and fungal communities in biological soil crusts and shrub root zone soils

“…First, the increased proportion of potassium metabolism genes in biocrusts compared to roots zones could represent an increased attempt by biocrust to access potassium where concentrations were lower (Table 1). Second, bacteria can employ KCl as an osmoprotectant which aids in surviving desiccation (Deole et al, 2013), and this result could reflect such a function, which would likely be a more stringent requirement for the interspace bacteria at the soil surface than the root zone bacteria. Third, potassium homeostasis may be linked to cyanobacterial photosynthesis, where potassium and a specific thylakoid potassium channel is required (Checchetto et al, 2012) and where potassium is required for recovery of photosynthesis upon rehydration (Qiu et al, 2004).…”

Life at high salt concentrations, intracellular KCl concentrations, and acidic proteomes

“…In view of their common phylogeny and documented content of organic osmotic solutes, the finding of an acidic proteome and of high intracellular KCl concentrations in H. halophila but not in H. halochloris (Deole et al, 2013) came as a big surprise. While the latter does not accumulate KCl, the first contains high KCl when grown at high salt (35%) but not at low salt (5%).…”

Dissection of Hexosyl- and Sialyltransferase Domains in the Bifunctional Capsule Polymerases from Neisseria meningitidis W and Y Defines a New Sialyltransferase Family

“…Interestingly, in this respect, adaptation to hypersaline environments has been shown to involve acidification of the proteome, the replacement of neutral amino acids with acidic residues to maintain hydration (41). Thus, one may speculate that nonulosonic acid biosynthesis pathways confer a selective advantage by similarly increasing the acidity of the glycome.…”