Abstract
Bacterial contact-dependent growth inhibition (CDI) is mediated by the CdiA/CdiB family of two-partner secretion proteins. Each CdiA protein exhibits a distinct growth inhibition activity, which resides in the polymorphic C-terminal region (CdiA-CT). CDI(+) cells also express unique CdiI immunity proteins that specifically block the activity of cognate CdiA-CT, thereby protecting the cell from autoinhibition. Here we show that many CDI systems contain multiple cdiA gene fragments that encode CdiA-CT sequences. These "orphan" cdiA-CT genes are almost always associated with downstream cdiI genes to form cdiA-CT/cdiI modules. Comparative genome analyses suggest that cdiA-CT/cdiI modules are mobile and exchanged between the CDI systems of different bacteria. In many instances, orphan cdiA-CT/cdiI modules are fused to full-length cdiA genes in other bacterial species. Examination of cdiA-CT/cdiI modules from Escherichia coli EC93, E. coli EC869, and Dickeya dadantii 3937 confirmed that these genes encode functional toxin/immunity pairs. Moreover, the orphan module from EC93 was functional in cell-mediated CDI when fused to the N-terminal portion of the EC93 CdiA protein. Bioinformatic analyses revealed that the genetic organization of CDI systems shares features with rhs (rearrangement hotspot) loci. Rhs proteins also contain polymorphic C-terminal regions (Rhs-CTs), some of which share significant sequence identity with CdiA-CTs. All rhs genes are followed by small ORFs representing possible rhsI immunity genes, and several Rhs systems encode orphan rhs-CT/rhsI modules. Analysis of rhs-CT/rhsI modules from D. dadantii 3937 demonstrated that Rhs-CTs have growth inhibitory activity, which is specifically blocked by cognate RhsI immunity proteins. Together, these results suggest that Rhs plays a role in intercellular competition and that orphan gene modules expand the diversity of toxic activities deployed by both CDI and Rhs systems.