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

The Candidate Phyla Radiation, also known as <i>Patescibacteria</i>, 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 <i>Patescibacteria</i> class <i>Saccharimonadia</i> as essential for their epibiotic growth. In this study, we used a comprehensive bioinformatics approach to investigate the diversity and distribution of T4SS within <i>Patescibacteria</i>. 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_FATA class of T4SS. This class is found in the conjugative elements of <i>Firmicutes</i>, <i>Actinobacteria</i>, <i>Tenericutes</i> and <i>Archaea</i>, indicating a possible horizontal gene transfer from these monoderm micro-organisms to <i>Patescibacteria</i>. We identified additional T4SS components near <i>virB4</i>, particularly those associated with the MPF_FATA class, as well as homologues of other T4SS classes, such as VirB2-like pilins, and observed their varied arrangements across different <i>Patescibacteria</i> classes. The absence of a relaxase in most of these T4SS clusters suggests that the system has been co-opted for other functions in <i>Patescibacteria</i>. The proximity of T4SS components to the origin of replication (gene <i>dnaA</i>) in some <i>Patescibacteria</i> 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 <i>Patescibacteria</i>.