Formation of flagella lacking outer rings by flaM, flaU, and flaY mutants of Escherichia coli.

Among flagellar mutants of Escherichia coli, flaM or flaU mutants form basal bodies lacking the outer P and L rings, whereas flaY mutants predominantly form basal bodies lacking the L ring. In these mutants, hooks and filaments are occasionally assembled onto these incomplete basal bodies. When the hook protein gene, flaFV, of Salmonella typhimurium was cloned on the multicopy plasmid pBR322 and introduced into these mutants, the efficiency with which cells assembled hooks and filaments onto the incomplete basal bodies increased significantly. Such cells formed characteristic dotted swarms on semisolid plates, indicating that cells carrying flagella without the outer rings are weakly motile because of poor function of their flagella, a low flagellar number per cell, or both of these defects. FlaV mutants also produced incomplete basal bodies lacking the outer rings, but assembly of hooks and filaments did not occur in these mutants even after introduction of the plasmid carrying flaFV of S. typhimurium. The failure in the case of flaV mutants was attributed to their inability to modify the rod tip to the structure competent for assembly of hook protein.