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. Bacteria that have this module inhibit or kill bacteria without it, giving them a growth advantage. Probably specifically inhibited by cognate immunity protein DddI (Probable). The C-terminal 163 residue fragment has double-stranded DNA cytidine deaminase activity; it does not deaminate ssDNA, ssRNA or dsRNA. Leads to C:G to T:A conversions in deaminated DNA. Preferentially deaminates 5'-TC-3' substrates (PubMed:32641830). a 2'-deoxycytidine in double-stranded DNA + H(+) + H2O = a 2'-deoxyuridine in double-stranded DNA + NH4(+) The toxic domain forms a 1:1 complex with the DddI immunity protein. The toxic domain is at the C-terminus (residues 1264-1427); expression of this domain in E.coli reduces viability nearly 1000-fold. A double dddA-dddI deletion has a 100-fold growth disadvantage compared to wild-type in competition experiments. The deaminase domain can be used to edit DNA. Is particularly useful for editing organellar DNA, as unlike CRISPR, it does not require guide RNA. Splitting the domain yields 2 halves that are not active until reassembled on target DNA by programmable DNA-binding proteins. Can be split at residue 1333 or 1397; each of the halves is then fused to a mitochondrial target signal, a TALE array specific for the targeted DNA, the half DddA toxin domain and a single UGI protein (uracil glycolase inhibitor), leading to editing without significant generation of insertions or deletions. Typical efficiencies on mitochondrial DNA (mtDNA) vary between 5 and 50% editing. It can be used in immortalized and non-immortalized cells, and in non-dividing cells as they continue to replicate mtDNA. A suspected tumor-related mutation in ND4 has been recreated using this system and shows the phenotypes expected for complex I disruption (PubMed:32641830). Has also been used to edit A.thaliana chloroplast (cp) DNA. The half deaminase domain is fused to a cp target signal, a TALE array specific for the targeted DNA, the half DddA toxin domain and a UGI protein in a T-DNA construct; plants were transformed by floral dipping. The process does not require tissue culture. Three cp genes were targeted (rrn16S, psbA, rpoC1). Homoplasmic substitutions were observed in many progeny and were stably inherited (PubMed:34211131). Belongs to the RHS/WapA nuclease family. Truncated N-terminus.