Crystal structure of the sensor domain of BaeS from Serratia marcescens FS14.

The sensor histidine kinases of two-component signal-transduction systems (TCSs) are essential for bacteria to adapt to variable environmental conditions. The two-component regulatory system BaeS/R increases multidrug and metal resistance in Salmonella and Escherichia coli. In this study, we report the X-ray structure of the periplasmic sensor domain of BaeS from Serratia marcescens FS14. The BaeS sensor domain (34-160) adopts a mixed alpha/beta-fold containing a central four-stranded antiparallel beta-sheet flanked by a long N-terminal alpha-helix and additional loops and a short C-terminal alpha-helix on each side. Structural comparisons revealed that it belongs to the PDC family with a remarkable difference in the orientation of the helix alpha 2. In the BaeS sensor domain, this helix is situated perpendicular to the long helix alpha 1 and holds helix alpha 1 in the middle with the beta sheet, whereas in other PDC domains, helix alpha 2 is parallel to helix alpha 1. Because the helices alpha 1 and alpha 2 is involved in the dimeric interface, this difference implies that BaeS uses a different dimeric interface compared with other PDC domains.