A widespread family of polymorphic contact-dependent toxin delivery systems in bacteria

Stephanie K. Aoki, et al. Nature 468: 439-442 (2010)

Speaker: Shu-Chun Chien (簡淑珺)                              Time: 15:10~16:00, Mar. 9, 2011

Commentator: Dr. Jiunn-Jong Wu (吳俊忠老師)        Place: Room 601

Abstract:

Contact-dependent growth inhibition (CDI), which is mediated by the CdiB/CdiA two-partner secretion system, was recently discovered in Escherichia coli. CDI inhibits the growth of other E. coli strains by cell-to-cell contact. To avoid autoinhibition, the bacterial cell also produces an immunity protein (CdiI). However, how CDI blocks cell growth and how CdiI counteracts the growth arrest are still unknown. In this study, the authors found that the amino acid sequence of CdiA carboxy-terminal region (CdiA-CT) is dissimilar among various E. coli strains. In order to determine the role of CdiA-CT, they grafted the CdiA-CT from other strains into the CdiA of strain UPUC536. They found that the bacterial cells expressing CdiI from different strains were protected from CDI mediated by chimaeric CdiA₅₃₆ proteins containing cognate, but not heterologous,CdiA-CT. They also deleted different regions of CdiA-CT and demonstrated that the growth inhibitory activity resided within the C-terminal 223 amino-acid residues of CdiA. Moreover, by pull-down assay and bacterial two-hybrid system they showed that CdiI immunity proteins could bind to their cognate CdiA-CTs in vitro and in vivo. By bioinformatic analysis and experiments they further found that many bacterial species possess CDI systems with high variability in the CdiA-CT and CdiI coding regions that determine various mechanisms of growth inhibition. Finally, the authors showed that lacking CdiA-CT and CdiI coding regions, the bacteria were unable to compete with the CDI⁺ cells in medium and on host cells. These results collectively suggest that the CDI systems represent a complex immunity network important for bacterial competition.

References

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