Identification of the nitrogen-fixing Pseudomonas stutzeri major flagellar gene regulator FleQ and its role in biofilm formation and root colonization

Flagellar biosynthesis and motility are subject to a four-tiered transcriptional regulatory circuit in Pseudomonas, and the master regulator FleQ appears to be the highest-level regulator in this hierarchical regulatory cascade. Pseudomonas stutzeri A1501 is motile by a polar flagellum; however, the motility and regulatory mechanisms involved in this process are unknown. Here, we searched the A1501 genome for flagella and motility genes and found that approximately 50 genes, which were distributed in three non-contiguous chromosomal regions, contribute to the formation, regulation and function of the flagella. The non-polar mutation of fleQ impaired flagellar biosynthesis, motility and root colonization but enhanced biofilm formation. FleQ positively regulates the expression of flagellar class II-IV genes, suggesting a regulatory cascade that is coordinated similar to that of the well-known P. aeruginosa. Based on our results, we propose that flagellar genes in P. stutzeri A1501 are regulated in a cascade regulated by FleQ and that flagellum-driven motility properties may be necessary for competitive rhizosphere colonization.