Abstract
Fluctuations in environmental osmolarity are ubiquitous stress factors in many natural habitats of microorganisms, as they inevitably trigger osmotically instigated fluxes of water across the semi-permeable cytoplasmic membrane. Under hyperosmotic conditions, many microorganisms fend off the detrimental effects of water efflux and the ensuing dehydration of the cytoplasm and drop in turgor through the accumulation of a restricted class of organic osmolytes, the compatible solutes. Ectoine and its derivative 5-hydroxyectoine are prominent members of these compounds and are synthesized widely by members of the Bacteria and a few Archaea and Eukarya in response to high salinity/osmolarity and/or growth temperature extremes. Ectoines have excellent function-preserving properties, attributes that have led to their description as chemical chaperones and fostered the development of an industrial-scale biotechnological production process for their exploitation in biotechnology, skin care, and medicine. We review, here, the current knowledge on the biochemistry of the ectoine/hydroxyectoine biosynthetic enzymes and the available crystal structures of some of them, explore the genetics of the underlying biosynthetic genes and their transcriptional regulation, and present an extensive phylogenomic analysis of the ectoine/hydroxyectoine biosynthetic genes. In addition, we address the biochemistry, phylogenomics, and genetic regulation for the alternative use of ectoines as nutrients.
Highlights
Microorganisms face myriad stressful conditions and nutrient limitations in their natural habitats; challenging circumstances to which they must react in a timely manner to ensure survival, persistence, and growth
We suggest that the envisioned EctC- and ectoine hydroxylases (EctD)-independent route for the synthesis of 5-hydroxyectoine [91] is of no physiological relevance in natural settings of osmotically stressed wild-type 5-hydroxyectoine-producing microorganisms
The EctC protein tree is dominated by ectoine synthases originating from Actinobacteria and from Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria, which together make up 91% of our dataset
Summary
Microorganisms face myriad stressful conditions and nutrient limitations in their natural habitats; challenging circumstances to which they must react in a timely manner to ensure survival, persistence, and growth. This strategy entails a rapid uptake of potassium ions as an emergency reaction to a sudden challenge by high osmolarity, but part of the initially amassed K+ pool is subsequently replaced by the cells through types of organic osmolytes that are highly compliant with cellular functions, the compatible solutes (Figure 1A) [1,2,3,4] In this way, the cell attains a level of hydration of the cytoplasm that is appropriate for biochemical processes and simultaneously upholds turgor without concurrently raising the intracellular ionic strength, as this would greatly impair most physiological activities of the cell [2,10]. The findings that some microorganisms combine an acidic proteome with the accumulation of compatible solutes [24,38,41,43], and that a substantial reduction in K+ content can be accomplished in salt-in adopters under more moderate salt-stress conditions [39], prompts the exploration of new avenues of research and raises intriguing questions about the role played by protein halophilicity in the evolution of microbial osmostress responses
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