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A Post-segregational Killing Mechanism for Maintaining Plasmid PMF1 in Its Myxococcus fulvus Host
Ya-Jie Li1, Ya Liu1, Zheng Zhang1
1State Key Laboratory of Microbial Technology, Institute of Microbial Technology, Shandong University, Qingdao, China.
Abstract:
Although plasmids provide additional functions for cellular adaptation to the environment, they also create a metabolic burden, which causes the host cells to be less competitive with their siblings. Low-copy-number plasmids have thus evolved several mechanisms for their long-term maintenance in host cells. pMF1, discovered in Myxococcus fulvus 124B02, is the only endogenous autonomously replicated plasmid yet found in myxobacteria. Here we report that a post-segregational killing system, encoded by a co-transcriptional gene pair of pMF1.19 and pMF1.20, is involved in maintaining the pMF1 plasmid in its host cells. We demonstrate that the protein encoded by pMF1.20 is a new kind of nuclease, which is able to cleave DNA in vitro. The nuclease activity can be neutralized by the protein encoded by pMF1.19 through protein-protein interaction, suggesting that the protein is an immune protein for nuclease cleavage. We propose that the post-segregational killing mechanism of the nuclease toxin and immune protein pair encoded by pMF1.20 and pMF1.19 is helpful for the stable maintenance of pMF1 in M. fulvus cells.
Insights
A novel post-segregational killing system involving a nuclease and an immune protein helps maintain the pMF1 plasmid in Myxococcus fulvus. This system ensures plasmid stability by eliminating cells that lose the plasmid.
Area of Science:
- Microbiology
- Molecular Biology
- Genetics
Background:
- Plasmids confer adaptive advantages but impose a metabolic burden on host cells.
- Low-copy-number plasmids require robust maintenance mechanisms for stable inheritance.
- pMF1 is the sole endogenous autonomously replicating plasmid identified in myxobacteria.
Purpose of the Study:
- To investigate the mechanisms responsible for the stable maintenance of the pMF1 plasmid in *Myxococcus fulvus*.
- To characterize the function of the gene pair *pMF1.19* and *pMF1.20* in plasmid stability.
Main Methods:
- Genetic analysis of the *pMF1.19* and *pMF1.20* gene pair.
- Biochemical assays to determine the enzymatic activity of the encoded proteins.
- In vitro DNA cleavage assays.
- Protein-protein interaction studies.
Main Results:
- A post-segregational killing system, encoded by *pMF1.19* and *pMF1.20*, was identified as crucial for pMF1 maintenance.
- The protein encoded by *pMF1.20* exhibits novel nuclease activity against DNA.
- The protein encoded by *pMF1.19* acts as an immune protein, neutralizing the nuclease activity via protein-protein interaction.
Conclusions:
- The identified nuclease-immune protein pair (*pMF1.20*/*pMF1.19*) constitutes a post-segregational killing system.
- This system is essential for the stable maintenance of the pMF1 plasmid in *M. fulvus*.
- The findings reveal a new mechanism for plasmid stability in bacteria.
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