Translation/secretion coupling by type III secretion systems.

Type III secretion systems mediate export of virulence proteins and flagellar assembly subunits in Gram-negative bacteria. Chaperones specific to each class of secreted protein are believed to prevent degradation of the secreted substrates. We show that an additional role of chaperones may be to regulate translation of secreted proteins. We show that the chaperone FIgN is required for translation of the flgM gene transcribed from one mRNA transcript (a flagellar class 3 transcript), but not from another (a flagellar class 2 transcript). FIgM translated from the class 3 transcript is primarily secreted whereas FIgM translated from the class 2 transcript is primarily retained in the cytoplasm. These results suggest FIgM and other type III secretion substrates possess both mRNA and amino acid secretion signals, and supports a new role for type III chaperones in translation/secretion coupling.