Abstract

The bacterial flagellum is one of the best-studied surface-attached appendages in bacteria. Flagellar assembly in vivo is promoted by its own protein export apparatus, a type III secretion system (T3SS) in pathogenic bacteria. Lysobacter enzymogenes OH11 is a non-flagellated soil bacterium that utilizes type IV pilus (T4P)-driven twitching motility to prey upon nearby fungi for food. Interestingly, the strain OH11 encodes components homologous to the flagellar type III protein apparatus (FT3SS) on its genome, but it remains unknown whether this FT3SS-like system is functional. Here, we report that, despite the absence of flagella, the FT3SS homologous genes are responsible not only for the export of the heterologous flagellin in strain OH11 but also for twitching motility. Blocking the FT3SS-like system by in-frame deletion mutations in either flhB or fliI abolished the secretion of heterologous flagellin molecules into the culture medium, indicating that the FT3SS is functional in strain OH11. A deletion of flhA, flhB, fliI, or fliR inhibited T4P-driven twitching motility, whereas neither that of fliP nor fliQ did, suggesting that FlhA, FlhB, FliI, and FliR may obtain a novel function to modulate the twitching motility. The flagellar FliI ATPase was required for the secretion of the major pilus subunit, PilA, suggesting that FliI would have evolved to act as a PilB-like pilus ATPase. These observations lead to a plausible hypothesis that the non-flagellated L. enzymogenes OH11 could preserve FT3SS-like genes for acquiring a distinct function to regulate twitching motility associated with its predatory behavior.

Highlights

  • Lysobacter enzymogenes is a Gram-negative, environmentally ubiquitous bacterium [1]

  • We further show that four FT3SS-like proteins (FlhA, FlhB, FliI, and FilR) in strain OH11 are required for T4P-driven twitching motility

  • Our results demonstrate that several FT3SS-like genes that are potentially vestigial in the non-flagellated L. enzymogenes appear to be required for T4P-driven twitching motility, highlighting the functional divergence of the FT3SS genes in flagellated and non-flagellated bacteria

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Summary

Introduction

Lysobacter enzymogenes is a Gram-negative, environmentally ubiquitous bacterium [1]. It was shown that this bacterium produces numerous anti-infectious metabolites and extracellular lytic enzymes [1,2,3,4]. A distinct feature of L. enzymogenes is the evolutionary loss of a surface-attached flagellum, due to the lack of multiple flagellar biogenesis genes such as the fliC gene encoding the flagellin subunit [5]. This non-flagellated bacterium exhibits a twitching behavior in natural niches that is powered by type IV pilus (T4P) [6]. In the model strain OH11, we previously discovered that numerous pilus structural component proteins, including the major pilus subunit, PilA and the motor proteins PilB, and the outer membrane secretin PilQ, are required for the biogenesis of T4P and the function of twitching motility [7]

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