| Summary: | Osmotic adaptation and accumulation of compatible solutes is a key process for life at high osmotic pressure and elevated salt concentrations. Most important solutes that can protect cell structures and metabolic processes at high salt concentrations are glycine betaine and ectoine. The genome analysis of more than 130 phototrophic bacteria shows that biosynthesis of glycine betaine is common among marine and halophilic phototrophic <i>Proteobacteria</i> and their chemotrophic relatives, as well as in representatives of <i>Pirellulaceae</i> and <i>Actinobacteria</i>, but are also found in halophilic <i>Cyanobacteria</i> and <i>Chloroherpeton thalassium.</i> This ability correlates well with the successful toleration of extreme salt concentrations. Freshwater bacteria in general lack the possibilities to synthesize and often also to take up these compounds. The biosynthesis of ectoine is found in the phylogenetic lines of phototrophic <i>Alpha</i>- and <i>Gammaproteobacteria</i>, most prominent in the <i>Halorhodospira</i> species and a number of <i>Rhodobacteraceae</i>. It is also common among <i>Streptomycetes</i> and <i>Bacilli</i>. The phylogeny of glycine-sarcosine methyltransferase (GMT) and diaminobutyrate-pyruvate aminotransferase (EctB) sequences correlate well with otherwise established phylogenetic groups. Most significantly, GMT sequences of cyanobacteria form two major phylogenetic branches and the branch of <i>Halorhodospira</i> species is distinct from all other <i>Ectothiorhodospiraceae</i>. A variety of transport systems for osmolytes are present in the studied bacteria.
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