Partitioning of the F plasmid: Overproduction of an essential protein for partition inhibits plasmid maintenance

Abstract

Multicopy plasmids carrying the sopB gene of the F plasmid inhibit stable inheritance of a coexisting mini-F plasmid. This incompatibility, termed IncG, is found to be caused by excess amounts of the SopB protein, which is essential for accurate partitioning of plasmid DNA molecules into daughter cells. A sopB-carrying multicopy plasmid that shows the IncG+ phenotype was mutagenized in vitro and IncG negative mutant plasmids were isolated. Among these amber and missense mutants of sopB, mutants with a low plasmid copy number and a mutant in the Shine-Dalgarno sequence for translation of the SopB protein were obtained. These results demonstrate that the IncG phenotype is caused by the SopB protein, and that the incompatibility is expressed only when the protein is overproduced. This suggests that the protein must be kept at appropriate concentrations to ensure stable maintenance of the plasmid.

Noncanonical SMC protein in Mycobacterium smegmatis restricts maintenance of Mycobacterium fortuitum plasmids

“…Taking the established evidence in the literature and our own experimental results together, we favor the second mechanism proposed above. However, we do not preclude the possibilities that EptC expression level (51,52) or an out-of-phase organization of the L and H sites of the pAL500 plasmid in the presence of EptC may secondarily contribute to plasmid loss.…”

“…The curvature is essential for the in-phase alignment of the L and H sites to form a looped structure (50). Based on knowledge from literature and our recent findings, several models for the mechanism of plasmid loss caused by EptC can be proposed: (i) Nonspecific DNA binding activity of EptC interferes with RepB binding; (ii) the alteration in supercoiling by EptC interferes with the in-phase alignment of the L and H sites or the ability of RepB to introduce long-range polar curvature; (iii) the expression level of partition proteins has been proved crucial for the faithful segregation in more than one scenario (51,52), and EptC disturbs the optimal expression level of partitioning proteins; and (iv) EptC competes with other SMCs to interact with topoisomerases, restricting the type of interaction necessary for segregation. It has been shown in E. coli that the correct supercoiling status is a prerequisite for plasmid segregation (17).…”

Structural Elements Required for Replication and Incompatibility of the Rhizobium etli Symbiotic Plasmid

“…In plasmids F and P1, an overexpression of polypeptides A or B induces incompatibility as a result of abnormal DNAprotein complexes formed between the A and B polypeptides and their respective centromerelike DNA sequences or by the overrepression that the A and B products exert on the transcription of their respective operons (1,8,13,23,29). In the R. etli symbiotic plasmid, the RepA product was identified as a trans-acting incompatibility determinant, because the reintroduction of extra copies of the repA gene, under the control of its own promoter, caused displacement of the symbiotic plasmid.…”

A single 43-bp sopC repeat of plasmid mini-F is sufficient to allow assembly of a functional nucleoprotein partition complex.

“…4). Function of the partition system is greatly influenced by the amount of Sop proteins present, and overexpression of sopB results in plasmid instability (3,28). The biological significance of the multiple 43-bp repeats in the wild-type sopC region is unknown, since even a single repeat appears to be sufficient for partition.…”