Conclusions Direct association of FliX and FlbD is required for their regulation on flagellar synthesis and other developmental events in Caulobacter. FliX and FlbD form high affinity complexes under physiological conditions, which is essential for the in vivo stability of each protein. Highly conserved regions of FliX are critical for binding to FlbD. Mutations in these regions could severely impact the recognition between the two and compromise their regulatory activity. Acknowledgements We are grateful to Dr. Jill Zeilstra-Ryalls at BGSU for helpful discussions.

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