Regulation of flagella

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Flagellar gene networks are fascinating, owing to their complexity — they usually coordinate the expression of more than 40 genes — and particular wiring that elicits temporal expression coupled to organelle morphogenesis. Moreover, many of the lessons learned from flagellar regulation are generally applicable to type III secretion systems. Our understanding of flagellar networks is rapidly expanding to include diverse organisms, as well as deepening to enable the development of predictive wiring diagrams. Numerous regulators control the regulation of flagella, and one of the next challenges in the field is to integrate flagellar gene control into master blueprints of global gene expression.

Introduction

Flagella are complex organelles capable of propelling bacteria through liquids (swimming) and through highly viscous environments or along surfaces (swarming). Many genes participate in generating this sort of motility system. The gene products are produced in a hierarchical fashion: the temporal pattern of gene expression, and/or protein production, generally conforms to the order in which the products are assembled. Figure 1 depicts the major parts of a flagellum. This review discusses only some of the many recent flagellar findings since 2002. The reader is also guided toward more in-depth reviews on various aspects of flagellar assembly and regulation [1, 2, 3], flagellar type three export [4, 5, 6], motor function [7] and swarming [8, 9, 10, 11, 12].

Section snippets

Transcriptional hierarchies

The most well understood flagella regulation is the transcription hierarchy found in Escherichia coli and Salmonella typhimurium; however, this paradigm, which has three tiers of gene control coordinated by the master regulators FlhD and FlhC, does not generalize to peritrichous (lateral) systems, or even beyond the enteric bacteria. In fact, some E. coli strains have been found to possess an additional flagellar system with a gene complement resembling the σ54-dependent lateral flagellar

Linking regulation to assembly

The beauty and complexity of flagellar assembly lies in how intimately gene regulation and protein supply are linked to substrate secretion. Built into all type III secretion and assembly systems that have been studied are checkpoints of control that couple temporal patterns of gene expression with organelle assembly or function. This strategy results in an ordered procession of substrates incorporated into, or exported by, the organelle and is achieved by numerous mechanisms. Some of the most

Regulating the master flagellar regulators

A rapidly expanding list of regulators acts to coordinate flagellar production. Some of the most recently investigated regulators are listed in Table 2. Many are response regulators, capable of integrating sensory input to the control of gene expression [34, 35, 36]. Many are global regulators [2, 11]; some are quorum-sensing responsive [37]; others are small RNAs — specifically, CsrB and CsrC, small RNAs that control the activity of the global regulator CsrA (RsmA) [38]. Regulation can also be

Conclusions: to swim, swarm or stick?

As evidenced by the numerous regulators influencing flagellar gene expression, the cell integrates multiple sensory inputs before committing to carefully programmed expression of flagellar genes. This program of flagellar gene expression has been particularly and elegantly dissected in E. coli and C. crescentus [54••, 55••]. One important and exciting consequence is that these flagellar gene networks can serve as models to study gene expression in silico. For E. coli, it has resulted in the

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

Work in my laboratory is supported by NSF grant MCB0315617.

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