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 ene rgizing 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 o f dominant and partially dominant motB missense mutations. Four of the 13 motB mutations yielded partially allele-specific suppressors. Of t he suppressing mutations, 57 are in the motA gene, eight are in fliG, and one is in fliM; no suppressor was identified in fliN. The prevalen ce 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 themselve s confer distinctive phenotypes in motB(+) cells. We propose a model i n which mutations affecting residues in or near the putative peptidogl ucan-binding region of MotB misalign the stator relative to the rotor. We suggest that most of the suppressors restore motility by introduci ng compensatory realignments in MotA or FliG. (C) 1996 Academic Press Limited