The type IV secretion system of is homologous to the bacterial monoderm conjugation machinery.

The Candidate Phyla Radiation, also known as , represents a vast and diverse division of bacteria that has come to light via culture-independent 'omics' technologies. Their limited biosynthetic capacity, along with evidence of their growth as obligate epibionts on other bacteria, suggests a broad reliance on host organisms for their survival. Nevertheless, our understanding of the molecular mechanisms governing their metabolism and lifestyle remains limited. The type IV secretion system (T4SS) represents a superfamily of translocation systems with a wide range of functional roles. T4SS genes have been identified in the class as essential for their epibiotic growth. In this study, we used a comprehensive bioinformatics approach to investigate the diversity and distribution of T4SS within . The phylogenetic analysis of the T4SS signature protein VirB4 suggests that most of these proteins cluster into a distinct monophyletic group with a shared ancestry to the MPF class of T4SS. This class is found in the conjugative elements of , , and , indicating a possible horizontal gene transfer from these monoderm micro-organisms to . We identified additional T4SS components near , particularly those associated with the MPF class, as well as homologues of other T4SS classes, such as VirB2-like pilins, and observed their varied arrangements across different classes. The absence of a relaxase in most of these T4SS clusters suggests that the system has been co-opted for other functions in . The proximity of T4SS components to the origin of replication (gene ) in some suggests a potential mechanism for increased expression. The broad ubiquity of a phylogenetically distinct T4SS, combined with its chromosomal location, underscores the significance of T4SS in the biology of .