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. The C-terminal 123 residues (954-1077) has DNase activity in the presence of Zn(2+), converting supercoiled DNA into open-circular form. Toxic activity is neutralized by coexpression of the cognate immunity protein CdiI-YPIII, but not by non-cognate immunity proteins from other toxin-immunity modules. Expression of the DNase domain as a chimera allows bacteria to attack other non-immune bacteria which become filamentous and have lost DNA staining. 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. Interacts with cognate immunity protein CdiI-YPIII, which blocks its toxic DNase activity. Encoded in a defective cdi locus that has been inactivated by gene rearragements and deletions.