Scientists from the Virginia Bioinformatics Institute (VBI) at Virginia Tech, the University of Maryland School of Medicine, and the University of Louisville have revealed that genes for a specific type of molecular secretion system in Rickettsia, a structure that is linked in many cases to virulence, have been conserved over many years of evolution.
The scientists compared the gene sequences of 13 Rickettsia species to detect a highly conserved type IV secretion system. Type IV secretion systems are membrane-spanning transporters that can act as syringes that inject virulence factors into the cells of their hosts (eukaryotes). Once introduced, these virulence factors compromise the host and may result in harmful disease, for example Legionnaires' disease (Legionella pneumophila) and Q fever (Coxiella burnetii). However, these secretion systems have not been implicated in human diseases caused by Rickettsia, including epidemic typhus (R. prowazekii) and Rocky Mountain spotted fever (R. rickettsii). Type IV secretion systems are unique in their ability to transport nucleic acids and proteins into plant and animal cells. A possible role of the transporter that is not directly associated with virulence, such as DNA transfer, has been overlooked in Rickettsia.
Dr. Joseph Gillespie, a bioinformatician at the Virginia Bioinformatics Institute and leader of the study, remarked: "We have used the Rickettsia genomic information steadily accumulated over the past 10 years as a starting point to look in detail at the origin and function of Rickettsia virulence-like genes. The study reveals a highly conserved type IV secretion system across the 13 genomes investigated, some of which are, intriguingly, not known to cause disease in their invertebrate and vertebrate hosts."