CitA (citrate) and DcuS (C4-dicarboxylate) sensor kinases in thermophilic Geobacillus kaustophilus and Geobacillus thermodenitrificans

The thermophilic Geobacillus thermodenitrificans and Geobacillus kaustophilus are able to use citrate or C-4-dicarboxylates like fumarate or succinate as the substrates for growth. The genomes of the sequenced Geobacillus strains (nine strains) each encoded a two-component system of the CitA family. The sensor kinase of G. thermodenitrificans (termed CitA(Gt)) was able to replace CitA of Escherichia coli (CitA(Ec)) in a heterologous complementation assay restoring expression of the CitA(Ec)-dependent citC-lacZ reporter gene and anaerobic growth on citrate. Complementation was specific for citrate. The sensor kinase of G. kaustophilus (termed DcuS(Gk)) was able to replace DcuS(Ec) of E coli. It responded in the heterologous expression system to C-4-dicarboxylates and to citrate, suggesting that DcuS(Gk) is, like DcuS(Ec), a C-4-dicarboxylate sensor with a side-activity for citrate. DcuS(Gk), unlike the homologous DctS from Bacillus subtilis, required no binding protein for function in the complementation assay. Thus, the thermophilic G. thermodenitrificans and G. kaustophilus contain citrate and C-4-dicarboxylate sensor kinases of the CitA and DcuS type, respectively, and retain function and substrate specificity under mesophilic growth conditions in E. coli.