Toxic component of a toxin-immunity protein module, which functions as a cellular contact-dependent growth inhibition (CDI) system. CDI modules allow bacteria to communicate with and inhibit the growth of closely related neighboring bacteria in a contact-dependent fashion. CDI is neutralized by its cognate immunity protein CdiI, but not by non-cognate CdiI from other bacteria. The C-terminal domain (CT) has strong DNase activity; this activity is inhibited by cognate CdiI. The CdiA protein is thought to be exported from the cell through the central lumen of CdiB, the other half of its two-partner system (TPS). The TPS domain probably remains associated with CdiB while the FHA-1 domain forms an extended filament with the receptor-binding domain (RBD) at its extremity; in the secretion arrested state the C-terminus of the RBD and YP domains form a hairpin-like structure as the FHA-2, PT and CT domains are periplasmic. The YP domain is probably responsible for this arrest at the point where it re-enters the host cell periplasm. Upon binding to a target cell outer membrane receptor a signal is transmitted to activate secretion. The filament elongates slightly, the rest of CdiA is secreted and the FHA-2 domain becomes stably associated with the target cell's outer membrane where it facilitates entry of the toxic CT domain into the target cell periplasm. From there the toxic CT domain is cleaved and gains access to the target cell cytoplasm via an inner membrane protein. The C-terminal (CT) domain interacts with cognate CdiI but not non-cognate CdiI from E.coli strain 536 / UPEC. In mixing experiments where target cells lack cdiI the DNase activity can be detected in target cells as a loss of DAPI staining (PubMed:23469034). Secreted to the cell surface by CdiB, its two partner secretion pathway (TPS) partner (Probable). Toxin translocation into the target cell depends on the proton motive force of the target cell, but not on tolA or tonB (PubMed:25174572). The CDI activity resides in the approximately 260 residue C-terminal (CT) domain; exchanging the C-terminal (CT) domain and cdiI gene between different strains confers resistance within cognate but not non-cognate systems (i.e. CdiI-3937 neutralizes CdiA-CT from 3937 but not CdiA-CT from E.coli strain 536 / UPEC or EC93 or Y.pestis CO92). There are 2 cdiBAI loci in this strain, this is locus 2. In the N-terminal section; belongs to the CdiA toxin family.