Bacterial genomes and plasmids encode a variety of peptide and protein toxins that mediate inter-bacterial competition. Bacteriocins are diffusible proteins that parasitize cell-envelope proteins to enter and kill bacteria. Contact-dependent growth inhibition (CDI) is one mechanism of inter-bacterial competition. Novel Toxin 21 (alternatively 16S rRNA endonuclease CdiA) belongs to a family of prokaryotic polymorphic toxin systems implicated in intra-specific conflicts PMID:24657090. This RNase toxin found in bacterial polymorphic toxin systems, is proposed to adopt the BECR (Barnase-EndoU-ColicinE5/D-RelE) fold, with two conserved lysine residues and [DS]xDxxxH, RxG[ST] and RxxD motifs. In bacterial polymorphic toxin systems, the toxin is usually exported by the type 2, type 4, type 5 or type 7 secretion systems PMID:22731697. This is also referred to as the E. cloacae CdiAC PMID:24657090. The CdiAC proteins carry a variety of sequence-diverse C-terminal domains, which represent a collection of distinct toxins PMID:22731697. Many CdiA-CT toxins have nuclease activities. In accord with the structural homology, CdiA-CT cleaves 16S rRNA at the same site as colicin E3 and this nuclease activity is responsible for growth inhibition PMID:24657090. These toxins are composed of three domains, each responsible for a distinct step in the cell-killing pathway. The central domain binds specific receptors on the surface of susceptible bacteria, the N-terminal domain mediates translocation across the cell envelope, and the C-terminal domain carries the bacteriocidal activity. This modular structure allows for delivery of diverse C-terminal toxins using conserved translocation and receptor-binding domains PMID:24657090.

PFAM:PF15526, INTERPRO:IPR028190,DOI:10.1016/j.str.2014.02.012