Guilty by association: Importers, exporters and MscS-type mechanosensitive channels encoded in biosynthetic gene clusters for the stress-protectant ectoine.

Abstract

Ectoine and its derivative hydroxyectoine are widely synthesized or imported by bacteria to fend off the detrimental effects of high osmolarity on cellular hydration and growth. Genes that are connected to a particular physiological process are often found in the same genomic context. We exploited this feature in a comprehensive bioinformatical analysis of 1103 ectoine biosynthetic gene clusters from Bacteria and Archaea through which we identified 415 ect operons that colocalize with genes encoding potential osmolyte transporters. These belong to various importer families. Focusing on the complex ect gene clusters of the alpha-proteobacteria Hyphomonas neptunium and Novoshingobium sp. LH128, we analysed several transporters with respect to their substrate specificities through physiological, molecular and modelling approaches. Accordingly, we identified an MFS-type uptake system specific for ectoines (EctU) and a novel SSS-type ectoine/hydroxyectoine importer (EctI) with a broader substrate profile for osmostress protectants. Furthermore, some ect gene clusters encode a MscS/YbdG-type mechanosensitive channel protein, whose functionality was assessed through down-shock assays. Moreover, our analysis identified the gene for the first putative ectoine/hydroxyectoine-specific efflux system (EctX), a member of the MFS superfamily. Our findings make substantial contributions to the understanding of the ecophysiology of ectoines, key players in microbial osmostress adjustment systems.