Abstract
Motility is finely regulated and is crucial to bacterial processes including colonization and biofilm formation. There is a trade-off between motility and growth in bacteria with molecular mechanisms not fully understood. Hypermotile Escherichia coli could be isolated by evolving non-motile cells on soft agar plates. Most of the isolates carried mutations located upstream of the flhDC promoter region, which upregulate the transcriptional expression of the master regulator of the flagellum biosynthesis, FlhDC. Here, we identified that spontaneous mutations in clpX boosted the motility of E. coli largely, inducing several folds of changes in swimming speed. Among the mutations identified, we further elucidated the molecular mechanism underlying the ClpXV78F mutation on the regulation of E. coli motility. We found that the V78F mutation affected ATP binding to ClpX, resulting in the inability of the mutated ClpXP protease to degrade FlhD as indicated by both structure modeling and in vitro protein degradation assays. Moreover, our proteomic data indicated that the ClpXV78F mutation elevated the stability of known ClpXP targets to various degrees with FlhD as one of the most affected. In addition, the specific tag at the C-terminus of FlhD being recognized for ClpXP degradation was identified. Finally, our transcriptome data characterized that the enhanced expression of the motility genes in the ClpXV78F mutations was intrinsically accompanied by the reduced expression of stress resistance genes relating to the reduced fitness of the hypermotile strains. A similar pattern was observed for previously isolated hypermotile E. coli strains showing high expression of flhDC at the transcriptional level. Hence, clpX appears to be a hot locus comparable to the upstream of the flhDC promoter region evolved to boost bacterial motility, and our finding provides insight into the reduced fitness of the hypermotile bacteria.
Highlights
Bacterial motility is one of the most extensively studied biological processes
We reported that adaptive evolution on soft agar could cause spontaneous mutations in clpX, which was an efficient way to boost bacterial motility, and supplied molecular reasons underlying the resulted hypermotile bacteria
The different effects of the evolved ClpX mutants on the motility observed in the present work and in the literature could depend on the features of strains applied
Summary
Bacterial motility is one of the most extensively studied biological processes. Flagellar motility helps bacteria to reach favorable environments (Fenchel, 2002), and bacterial flagella play a crucial role in biofilm formation (Guttenplan and Kearns, 2013) and colonization (Ageorges et al, 2020). The biosynthesis and operation of flagella are energy-consuming processes, costing 2% of the biosynthetic energy in E. coli (Soutourina and Bertin, 2003; Fontaine et al, 2008). There is a trade-off between motility and growth, and bacteria need to coordinate flagellum biosynthesis well with cellular growth. As a result, both the synthesis and the function of flagella are finely tuned
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