flhDC, but not fleQ, regulates flagella biogenesis in Azotobacter vinelandii, and is under AlgU and CydR negative control

Abstract

Azotobacter vinelandii is a nitrogen-fixing soil bacterium that undergoes differentiation to form cysts resistant to desiccation. Upon encystment, this bacterium becomes non-motile. As in enteric bacteria, motility in A. vinelandii occurs through the use of peritrichous flagella. Pseudomonas aeruginosa, a phylogenetically close relative of A. vinelandii, possesses a single polar flagellum. The FlhDC proteins are the master regulators of flagella and motility in enterobacteria, whereas FleQ is the master regulator in P. aeruginosa, and it is under AlgU (sigmaE) negative control. At present, nothing is known about the organization and expression of flagella genes in A. vinelandii. Here, we identified the flagella gene cluster of this bacterium. Homologues of the master regulatory genes flhDC and fleQ are present in A. vinelandii. Inactivation of flhDC, but not fleQ, impaired flagella biogenesis and motility. We present evidence indicating that a negative effect of the AlgU sigma factor on flhDC expression causes loss of motility in A. vinelandii, and that CydR (a homologue of Fnr) is under AlgU control and has a negative effect on flhDC expression. Taken together, these results suggest the existence of a cascade consisting of AlgU and CydR that negatively controls expression of flhDC; the results also suggest that the block in flagella synthesis under encystment conditions centres on flhDC repression by the AlgU–CydR cascade.

Characterization of UDP-Glucose Dehydrogenase and UDP-Glucose Pyrophosphorylase Mutants ofProteus mirabilis: Defectiveness in Polymyxin B Resistance, Swarming, and Virulence

“…Therefore, activation of RpoE caused by ugd or galU mutation could explain why the ugd or galU mutant is defective in swarming and virulence factor expression. The observation that RpoE can negatively regulate FlhDC expression has been reported previously for Azotobacter (33).…”

NsrR targets in the Escherichia coli genome: new insights into DNA sequence requirements for binding and a role for NsrR in the regulation of motility

“…In the nonpathogenic organism Nitrosomonas europaea, NO stimulates biofilm formation (Schmidt et al, 2004). In Azotobacter vinelandii, expression of the flhDC genes (which encode the master regulator of motility) is negatively regulated by the oxygen-sensor CydR, an orthologue of the E. coli FNR protein (León and Espín, 2008). CydR is sensitive to NO (Wu et al, 2000), suggesting that exposure to NO might stimulate motility in A. vinelandii via increased expression of flhDC.…”

Genome Sequence ofAzotobacter vinelandii, an Obligate Aerobe Specialized To Support Diverse Anaerobic Metabolic Processes

“…for CydR in several other metabolic processes, including synthesis of PHB (95) and flagellar motility (48). Although the role of CydR in respiration is not completely defined, its direct or indirect participation in other cellular processes is even more obscure.…”