Full text loading...
Chapter 8 : Type IV Secretion Systems
Type IV transporters differ from other transporter systems in that type IV systems are used not only to transport proteins but also to mobilize DNA. In fact, it is likely that type IV transporters first evolved as conjugation systems that functioned to transfer genetic information between bacteria and only later were these systems modified by pathogenic bacteria to transfer critical virulence factors across the bacterial membranes and into the host eukaryotic cell. Since transfer of genetic information from one bacterium to another via bacterial conjugation is an ancient system that likely predates the evolution of pathogens and therefore predates the necessity to transport virulence factors across bacterial membranes, it seems likely that the first type IV transporter was a conjugation system that transferred DNA from one bacterium to another. A slight variation of type IV DNA transport systems appears to have occurred when type IV systems evolved that solely transport proteins without any DNA attached. Pathogens that produce type IV transporters include Bordetella pertussis, Brucella spp., Bartonella henselae, Helicobacter pylori, Rickettsia prowazekii, and Legionella pneumophila. B. pertussis is the causative agent of the disease pertussis, or whooping cough. This pathogen secretes one of its important virulence factors, pertussis toxin (PT), using a type IV transporter. Several intracellular pathogens utilize type IV transporters to export important virulence factors from the bacterium to the cellular milieu of the host cell.