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 C4-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 CitAGt) was able to replace CitA of Escherichia coli (CitAEc) in a heterologous complementation assay restoring expression of the CitAEc-dependent citC-lacZ reporter gene and anaerobic growth on citrate. Complementation was specific for citrate. The sensor kinase of G. kaustophilus (termed DcuSGk) was able to replace DcuSEc of E. coli. It responded in the heterologous expression system to C4-dicarboxylates and to citrate, suggesting that DcuSGk is, like DcuSEc, a C4-dicarboxylate sensor with a side-activity for citrate. DcuSGk, 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 C4-dicarboxylate sensor kinases of the CitA and DcuS type, respectively, and retain function and substrate specificity under mesophilic growth conditions in E. coli.