Transcriptional Hierarchy of Aeromonas hydrophila Polar-Flagellum Genes

Abstract

Aeromonas hydrophila polar-flagellum class I gene transcription is 70 dependent, which is consistent with the fact that the A. hydrophila polar flagellum is constitutively expressed. In contrast to other bacteria with dual flagellar systems such as Vibrio parahaemolyticus, the A. hydrophila LafK protein does not compensate for the lack of the polar-flagellum regulator FlrA (V. parahaemolyticus FlaK homologue). This is consistent with the fact that the A. hydrophila FlrA mutation abolishes polar-flagellum formation in liquid and on solid surfaces but does not affect inducible lateral-flagellum formation. The results highlight that the polar-and lateralflagellum interconnections and control networks are specific and that there are differences between the dual flagellar systems in A. hydrophila and V. parahaemolyticus. Furthermore, our results indicate that the A. hydrophila polar-flagellum transcriptional hierarchy (also in class II, III, and IV genes) shares some similarities with but has many important differences from the transcriptional hierarchies of Vibrio cholerae and Pseudomonas aeruginosa. The A. hydrophila flhF and flhG genes are essential for the assembly of a functional polar flagellum because in-frame mutants fail to swim in liquid medium and lack the polar flagellum. In Vibrio and Pseudomonas flhG disruption increases the number of polar flagella per cell, and Pseudomonas flhF disruption gives an aberrant placement of flagellum. Here, we propose the gene transcriptional hierarchy for the A. hydrophila polar flagellum.Flagellum motility represents an important advantage for bacteria in moving toward favorable conditions or in avoiding detrimental environments, and it allows flagellated bacteria to successfully compete with other microorganisms (14). Flagellum morphogenesis is a complex cascade of events that requires coordinate expression of more than 50 genes encoding structural subunits, regulatory proteins, and chemo-sensor machinery. These genes have been categorized in relation to their temporal requirement during the assembly process into three groups: early, middle, and late genes (1, 10). Early genes encode regulatory proteins that control the expression of the entire regulon. Middle genes include structural components of the hook, the basal body, the export apparatus, and regulatory proteins that couple late-gene expression; and late genes include the filament, motor force generators, and chemotactic proteins. The expression of these genes is an energetically expensive process for the bacterium, and all flagellar systems are highly regulated. Regulation of flagellum biogenesis involves a combination of transcriptional, translational, and posttranslational mechanisms (1). In relation to their transcriptional hierarchy, the flagellar clusters of different bacterial species are transcribed from three different promoter classes, whose differential expression is coordinated by the activity of transcriptional regulators which include alternative sigma factors and anti-sigma factors (10, 25). The co...

The GTPase Activity of FlhF Is Dispensable for Flagellar Localization, but Not Motility, in Pseudomonas aeruginosa

“…The structure of the bacterial flagellum is well described (3). The regulation of flagellar biosynthesis is less well understood, and different models have been proposed for the hierarchy of flagellar assembly in diverse bacteria (4)(5)(6).…”

Aeromonas hydrophila Lateral Flagellar Gene Transcriptional Hierarchy

“…A. hydrophila and V. parahaemolyticus both have dual flagellar systems (polar and lateral flagella) but do not share structural or regulatory genes, and both contribute to motility in semisolid plates (9,18,19). A. hydrophila polar flagellum is constitutive; however, lateral flagella are induced in highly viscous media or on surfaces.…”

“…One of the class II clusters encodes a twocomponent signal-transducing system (FlrBC in A. hydrophila) whose regulator (FlrC) activates class III genes. The class III-transcribed A. hydrophila 28 factor, which activates transcription of class IV genes, is class II transcribed in Vibrio spp., and flagellar hierarchy is independently transcribed in P. aeruginosa (16,17,18).Inducible peritrichous flagella (lateral flagella) of V. parahaemolyticus and A. hydrophila do not posses an FlhDC master regulator and are 54 dependent (9, 19) as polar flagella. In this work, we investigated the A. hydrophila lateral flagellar transcriptional hierarchy by two techniques: promoter-lacZ fusion assays (used to measure ␤-galactosidase activity in several mutant backgrounds) and reverse transcription-PCR (RT-PCR) assays.…”

“…In polar flagellated Gammaproteobacteria, such as Vibrio, mesophilic Aeromonas, and Pseudomonas species, four levels of hierarchy have been described. Transcription of class II and III is 54 dependent, and transcription of class IV is 28 dependent (16,17,18). At the top of the hierarchy is a 54 -associated transcriptional activator (FlrA in A. hydrophila) which activates 54 -dependent promoters preceding the class II clusters.…”

“…One of the class II clusters encodes a twocomponent signal-transducing system (FlrBC in A. hydrophila) whose regulator (FlrC) activates class III genes. The class III-transcribed A. hydrophila 28 factor, which activates transcription of class IV genes, is class II transcribed in Vibrio spp., and flagellar hierarchy is independently transcribed in P. aeruginosa (16,17,18).…”