Length of the flagellar hook and the capacity of the type III export apparatus

Length determination in biology generally uses molecular rulers. The hook, a part of the flagellum of motile bacteria, has an invariant Length. Here, we examined hook length and found that it was determined not by molecular r ulers but probably by the amount of subunit protein secreted by the flagell ar export apparatus. The export apparatus shares common features with the t ype III virulence-factor secretion machinery and thus may be used more wide ly in Length determination of structures other than flagella.