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Concept
Type III secretion system
Summary
The type III secretion system (T3SS or TTSS), also called the injectisome, is one of the bacterial secretion systems used by bacteria to secrete their effector proteins into the host's cells to promote virulence and colonisation. The T3SS is a needle-like protein complex found in several species of pathogenic gram-negative bacteria.
The term Type III secretion system was coined in 1993. This secretion system is distinguished from at least five other secretion systems found in gram-negative bacteria. Many animal and plant associated bacteria possess similar T3SSs. These T3SSs are similar as a result of convergent evolution and phylogenetic analysis supports a model in which gram-negative bacteria can transfer the T3SS gene cassette horizontally to other species. The most researched T3SSs are from species of Shigella (causes bacillary dysentery), Salmonella (typhoid fever), Escherichia coli (Gut flora, some strains cause food poisoning), Vibrio (gastroenteritis and diarrhea), Burkholderia (glanders), Yersinia (plague), Chlamydia (sexually transmitted disease), Pseudomonas (infects humans, animals and plants) and the plant pathogens Erwinia, Ralstonia and Xanthomonas, and the plant symbiont Rhizobium.
The T3SS is composed of approximately 30 different proteins, making it one of the most complex secretion systems. Its structure shows many similarities with bacterial flagella (long, rigid, extracellular structures used for motility). Some of the proteins participating in T3SS share amino-acid sequence homology to flagellar proteins. Some of the bacteria possessing a T3SS have flagella as well and are motile (Salmonella, for instance), and some do not (Shigella, for instance). Technically speaking, type III secretion is used both for secreting infection-related proteins and flagellar components. However, the term "type III secretion" is used mainly in relation to the infection apparatus. The bacterial flagellum shares a common ancestor with the type III secretion system.
T3SSs are essential for the pathogenicity (the ability to infect) of many pathogenic bacteria.
Official source
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