Translation/secretion coupling by type III secretion systems

Type III secretion systems mediate export of virulence proteins and flagell ar assembly subunits in Gram-negative bacteria. Chaperones specific to each class of secreted protein are believed to prevent degradation of the secre ted substrates. We show that an additional role of chaperones may be to reg ulate translation of secreted proteins. We show that the chaperone FlgN is required for translation of the flgM gene transcribed from one mRNA transcr ipt (a flagellar class 3 transcript), but not from another (a flagellar cla ss 2 transcript). FlgM translated from the class 3 transcript is primarily secreted whereas FlgM translated from the class 2 transcript is primarily r etained in the cytoplasm. These results suggest FlgM and other type III sec retion substrates possess both mRNA and amino acid secretion signals, and s upports a new role for type III chaperones in translation/secretion couplin g.