The NtrB/C two-component system positively regulates the motility in Serratia marcescens FS14

NtrBC is well characterized for its regulation in nitrogen source utilization in bacteria, and also involved in the regulation of virulence and motility which is crucial for ecological adaptation and potential relevance to virulence. Here, the role of NtrBC was investigated in Serratia marcesencs FS14. NtrB was found to be essential for both swarming and swimming motility of FS14 whereas deletion of ntrC only slightly impaired the motility. Transcriptional analysis and EMSA demonstrated that NtrBC positively regulates the expression of both flagellar regulatory gene flhDC and the surfactant synthesis gene swrW by directly binding to the promoters of these genes with NtrC. Kinase activity of NtrB and phosphorylation of NtrC were further demonstrated to be critical for the motility regulation. Phenotypic discrepancy between ntrC^D54A, ntrC deletion mutant and ntrC^D54E,S161F with EMSA results suggested that there is probably another regulatory component involved in the regulation of bacterial motility which is most likely using the same cis element as NtrC with relative lower binding affinity. Comparison with the motility regulation results of NtrBC in Pseudomonas aeruginosa and Burkholderia cenocepacia H111, our results suggested that NtrBC may modulate the motility by a new and complicated regulatory mode in S. marcesence FS14.