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immunity protein of the contact-dependent growth inhibition (CDI) system of Escherichia coli NC101, and similar proteins CDI toxins are expressed by gram-negative bacteria as part of a mechanism to inhibit the growth of neighboring cells. This model represents the inhibitor (CdiI, also called CdiI immunity protein) of the CdiA effector protein from Escherichia coli NC101. CdiA secretion is dependent on the outer membrane protein CdiB. Upon binding to a receptor on the surface of target bacteria, the CDI toxin is delivered via its C-terminal domain (CdiA-CT). CdiI immunity proteins are intracellular proteins that inactivate the toxin/effector protein. They are specific for their cognate CdiA-CT and do not protect cells from the toxins of other CDI+ bacteria. The CdiI immunity protein binds the CdiA toxin via its C-terminal domain to prevent auto-inhibition. Thus, CDI systems encode a complex network of toxin-immunity protein pairs that are deployed for intercellular competition. The E. coli NC101 CdiI exerts a unique ribonuclease activity that cleaves the single-stranded 3'-end from tRNAs that contain guanine discriminator nucleotides; for this, it requires elongation factor Tu (EF-Tu), an essential and highly conserved translation factor that delivers aminoacyl-tRNAs (aa-tRNA) to the ribosome during protein synthesis. Structure studies suggest the toxin remodels the GTP:EF-Tu:aa-tRNA complexes to free the 3'-end of aa-tRNA for entry into the nuclease active site. |
Jiyao W et al.(2023). "The conserved domain database in 2023.",