Genetic analysis of assembly of the Salmonella enterica serovar typhimurium type III secretion-associated needle complex

Several pathogenic bacteria have evolved a specialized protein secretion sy stem termed type III to secrete and deliver effector proteins into eukaryot ic host cells. Salmonella enterica serovar Typhimurium uses one such system to mediate entry into nonphagocytic cells. This system is composed of more than 20 proteins which are encoded within a pathogenicity island (SPI-1) l ocated at centisome 63 of its chromosome. A subset of these components form a supramolecular structure, termed the needle complex, that resembles the flagellar hook-basal body complex The needle complex is composed of a multi ple-ring cylindrical base that spans the bacterial envelope and a needle-li ke extension that protrudes from the bacterial outer surface. Although the components of this structure have been identified, little is known about it s assembly. In this study we examined the effect of loss-of-function mutati ons in each of the type III secretion-associated genes encoded within SPI-1 on the assembly of the needle complex. This analysis indicates that the as sembly of this organelle occurs in discrete, genetically separable steps. A model for the assembly pathway of this important organelle is proposed tha t involves a sec-dependent step leading to the assembly of the base substru cture followed by a sec-independent process resulting in the assembly of th e needle portion.