Characterization of RcgA and RcgR, two rhizobial proteins involved in the modulation of plasmid transfer

Plasmid transfer is a central mechanism of gene exchange in bacteria, enabling the spread of traits with ecological and evolutionary relevance. <i>Rhizobium favelukesii</i> is a soil bacterium that carries multiple plasmids, including pLPU83a, which serves as a model to study conjugative transfer. This plasmid requires the transcriptional regulator TraR for transfer but-unlike classical systems-lacks the cognate gene that encodes the AHL synthase typically involved in quorum-sensing regulation. In previous work, two novel proteins encoded on pLPU83a, RcgA and RcgR, were identified as key elements in this regulatory system. Here, we further characterized their roles: RcgR represses the transcription of <i>traR</i> and, consequently, that of all conjugative genes, while RcgA is essential for transfer and localizes to the membrane, suggesting a structural function. These results provide mechanistic insight into how plasmid transfer is regulated in systems uncoupled from quorum sensing, highlighting alternative layers of control in bacterial conjugation.