Mutations in motB suppressible by changes in stator or rotor components of the bacterial flagellar motor

Five proteins (MotA, MotB, FliG, FliM and FliN) may be involved in energizing flagellar rotation in Escherichia coli. To study interactions between the Mot proteins, and between them and the three Fli proteins of the switch-motor complex, we have isolated extragenic suppressors of dominant and partially dominant motB missense mutations. Four of the 13 motB mutations yielded partially allele-specific suppressors. Of the suppressing mutations, 57 are in the motA gene, eight are in fliG, and one is in fliM; no suppressor was identified in fliN. The prevalence of suppressors in fliG suggests that FliG interacts rather directly with the Mot proteins. The behaviour of cells in tethering and swarm assays indicates that the motA suppressors are more efficient than the fliG or fliM suppressors. Some of the suppressing mutations themselves confer distinctive phenotypes in motB+ cells. We propose a model in which mutations affecting residues in or near the putative peptidoglucan-binding region of MotB misalign the stator relative to the rotor. We suggest that most of the suppressors restore motility by introducing compensatory realignments in MotA or FliG.