EXTRAGENIC SUPPRESSION OF MOTA MISSENSE MUTATIONS OF ESCHERICHIA-COLI

The MotA and MotB proteins are thought to comprise elements of the sta tor component of the flagellar motor of Escherichia coli. In an effort to understand interactions among proteins within the motor, we attemp ted to identify extragenic suppressors of 31 dominant, plasmid-borne a lleles of motA. Strains containing these mutations were either nonmoti le or had severely impaired motility. Four of the mutants yielded extr agenic suppressors mapping to the FlaII or FlaIIIB regions of the chro mosome. Two types of suppression were observed. Suppression of one typ e (class I) probably results from increased expression of the chromoso mal motB gene due to relief of polarity, Class I suppressors were part ial deletions of Mu insertion sequences in the disrupted chromosomal m otA gene, Class I suppression was mimicked by expressing the wild-type MotB protein from a second, compatible plasmid. Suppression of the ot her type (class II) was weaker, and it was not mimicked by overproduct ion of wild-type MotB protein, Class II suppressors were point mutatio ns in the chromosomal motB or fliG genes. Among 14 independent class I I suppressors characterized by DNA sequencing, we identified six diffe rent amino acid substitutions in MotB and one substitution in FliG. A number of the strongest class II suppressors had alterations of residu es 136 to 138 of MotB, This particular region within the large, C-term inal periplasmic domain of MotB has previously not been associated wit h a specific function. We suggest that residues 136 to 138 of MotB may interact directly with the periplasmic face of MotA or help position the N-terminal membrane-spanning helix of MotB properly to interact wi th the membrane-spanning helices of the MotA proton channel.