Abstract
Extremely halophilic microorganisms that accumulate KCl for osmotic balance (the Halobacteriaceae, Salinibacter) have a large excess of acidic amino acids in their proteins. This minireview explores the occurrence of acidic proteomes in halophiles of different physiology and phylogenetic affiliation. For fermentative bacteria of the order Halanaerobiales, known to accumulate KCl, an acidic proteome was predicted. However, this is not confirmed by genome analysis. The reported excess of acidic amino acids is due to a high content of Gln and Asn, which yield Glu and Asp upon acid hydrolysis. The closely related Halorhodospira halophila and Halorhodospira halochloris use different strategies to cope with high salt. The first has an acidic proteome and accumulates high KCl concentrations at high salt concentrations; the second does not accumulate KCl and lacks an acidic proteome. Acidic proteomes can be predicted from the genomes of some moderately halophilic aerobes that accumulate organic osmotic solutes (Halomonas elongata, Chromohalobacter salexigens) and some marine bacteria. Based on the information on cultured species it is possible to understand the pI profiles predicted from metagenomic data from hypersaline environments.
Highlights
In a study of the proteins of Halobacterium and Halococcus, Reistad (1970) noted an unusual amino acids composition of the cells’ bulk protein: a great excess of the acidic amino acids glutamate and aspartate compared to the basic amino acids lysine and arginine
The acidic proteins of the Halobacteriaceae typically require high salt concentrations for structural stability and activity, and the presence of such an acidic proteome was considered to be correlated with the accumulation of molar concentrations of KCl to provide osmotic balance to the cells (Lanyi, 1974; Mevarech et al, 2000; Oren, 2013b)
A different strategy of osmotic adaptation in halophilic and halotolerant microorganisms is the accumulation of organic osmotic solutes
Summary
Halophilic microorganisms that accumulate KCl for osmotic balance (the Halobacteriaceae, Salinibacter ) have a large excess of acidic amino acids in their proteins. This minireview explores the occurrence of acidic proteomes in halophiles of different physiology and phylogenetic affiliation. For fermentative bacteria of the order Halanaerobiales, known to accumulate KCl, an acidic proteome was predicted. This is not confirmed by genome analysis. Acidic proteomes can be predicted from the genomes of some moderately halophilic aerobes that accumulate organic osmotic solutes (Halomonas elongata, Chromohalobacter salexigens) and some marine bacteria.
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