KdpE of Clostridium acetobutylicum is a highly specific response regulator controlling only the expression of the kdp operon.

KdpE from Clostridium acetobutylicum was enriched in form of its Strep-tag-derivative to allow easy immunodetection. It could be artificially phosphorylated by acetyl phosphate or carbamyl phosphate. Only phosphorylated clostridial KdpE was able to bind to a region upstream of the clostridial kdp structural genes. The minimal sequence requirements for binding were determined and found to share significant similarity with the Escherichia coli KdpE binding motif. However, the clostridial protein proved to be much more specific and did not bind in unphosphorylated form or to other similar sequences either from C. acetobutylicum or E. coli. In contrast, the enterobacterial protein recognized the clostridial binding motif. An HPt domain has been detected in KdpD from C. acetobutylicum, the cognate sensor kinase of KdpE. The data reported indicate that in E. coli, KdpE might represent a regulatory checkpoint for different phosphorelay signalling pathways, whereas in C. acetobutylicum KdpD might serve this function.