Nonsense motility mutants in Salmonella typhimurium.

Of 313 motility-deficient mutants isolated from an LT2 his(amber) strain fixed in phase 1 by gene vh2(-), 25 regained motility when amber or ochre suppressors were introduced, in F' factors or by transduction. The fla mutants (23 amber, 1 ochre) fell in complementation groups A, B, C, F, K, a new group, M, and at least one further new group; the hypothesis of a fla gene which specifies only an RNA structural component of a flagellum-synthesizing basal apparatus is disproven for the corresponding genes. Hfr and transductional crosses confirmed gene assignments from complementation and indicated that flaM and another new fla locus map near H1. A small minority of motile bacteria were detectable in many of the amber fla mutants. In groups A and F some pairs of amber fla mutants complemented each other, and perhaps each of these groups corresponds to more than one structural gene. The suppressed derivatives of a mutant with an amber mutation in H1 made flagella morphologically and serologically indistinguishable from wild-type flagella. A slow-spreading but flagellate mutant showed mainly non-translational motility in broth, and in a viscous medium the bacteria reversed very frequently; its amber mutation, probably near H1, is inferred to cause a defect in chemotaxis, so that the bacteria give the avoidance reaction continuously.