The C-terminal half of the anti-sigma factor FlgM contains a dynamic equilibrium solution structure favoring helical conformations.

FlgM is the inhibitor of sigma 28, a transcription factor specific for the expression of bacterial flagella and chemotaxis genes. FlgM is also exported from the cytoplasm to the outside of the cell during the process of flagella filament assembly. In the absence of its targets, FlgM is a dynamic, mostly unfolded, molecule [Daughdrill, G. W., et al. (1997) Nat. Struct. Biol. 4(4), 285-291]. The NMR resonance assignments, dynamics, and average secondary structure of this mostly unfolded form of FlgM are reported here. Because of the dynamic behavior of FlgM, the deviation of C alpha chemical shifts from the random coil values was used to test for the presence of secondary structure [Wishart, D. S., and Sykes, B. D. (1994) Methods Enzymol. 239, 363-392]. This analysis shows two contiguous regions in the C-terminal half of FlgM with helical C alpha chemical shifts. These two regions, M60-G73 and A83-A90, contained less than 10 medium-range NOEs, and the 15N relaxation parameters suggest the helical structure is not rigid. However, the C alpha chemical shifts of M60-G73, A83-A90, and other residues in the C-terminal half of FlgM shift toward their canonical random coil values with the addition of a chemical denaturant. Along with the values of the order parameter, S2, this observation suggests the C-terminal half of FlgM exists in an equilibrium structural state that is nonrandom. The same analysis of the N-terminal half of FlgM suggests it more closely resembles a random coil in conditions with and without denaturant. It appears the C-terminal half of FlgM lacks sufficient intramolecular contacts to form stable secondary or tertiary structures. It is known this C-terminal region becomes rigidly held when FlgM binds sigma 28 (Daughdrill et al., 1997), and it is possible that binding stabilizes the helical structure. The potential evolutionary relationship between the inhibitory interaction of FlgM with sigma 28 and the autoinhibition observed in sigma 70 is discussed.