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MobB is a small protein encoded by the broad-host-range plasmid R1162 and required for efficient mobilization of its DNA during conjugation. The protein was shown previously to stabilize the relaxosome, the complex of plasmid DNA and mobilization proteins at the origin of transfer (oriT). We have generated in-frame mobB deletions that specifically inactivate the stabilizing effect of MobB while still allowing a high rate of transfer. Thus, MobB has two genetically distinct functions in transfer. The effect of another deletion, extending into mobA, indicates that both functions require a specific region of MobA protein that is distinct from the nicking-ligating domain. The mobB mutations that specifically affected stability also resulted in poor growth of cells, due to increased transcription from the promoters adjacent to oriT. The effects of the mutations could be suppressed not only by full-length MobB provided in trans, as expected, but also by additional copies of oriT, cloned in pBR322. In addition, in the presence of MobA both the full-length and truncated forms of MobB stimulated recombination between oriT-containing plasmids. We propose a model in which MobB regulates expression of plasmid genes by altering the stability of the relaxosome, in a manner that involves the coupling of plasmid molecules.

Stabilization of the Relaxosome and Stimulation of Conjugal Transfer Are Genetically Distinct Functions of the R1162 Protein MobB