A Post-segregational Killing Mechanism for Maintaining Plasmid PMF1 in Its Myxococcus fulvus Host.

Ya-jie Li,Ya Liu,Ya Gong,Yue-zhong Li,Zheng Zhang,Xiao-jing Chen

doi:10.3389/fcimb.2018.00274

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.

Highlights

Plasmids are self-replicating extra-chromosomal genetic elements that are dispensable to their bacterial host cells
We propose that together the two genes play a major role in the stable maintenance of plasmid pMF1 in M. fulvus cells using a post-segregational killing mechanism
BLASTp searching against GenBank databases showed that the predicted amino acid sequences of many genes in the pMF1 plasmid had homologs in various myxobacterial genome sequences (Table S1), all of which were functionally undetermined

Summary

INTRODUCTION

Plasmids are self-replicating extra-chromosomal genetic elements that are dispensable to their bacterial host cells. They usually contain some essential genes for independent replication and segregation (Dmowski and Jagura-Burdzy, 2013), and some accessory genes (Hulter et al, 2017). The presence of plasmids leads to extra metabolic burdens, which cause the host cells to be less competitive with their siblings. We previously isolated pMF1, which is the only endogenous plasmid yet found in myxobacteria, from Myxococcus fulvus 124B02 (Zhao et al, 2008). The presence of the gene pair made plasmids more stable in Myxococcus cells. We propose that together the two genes play a major role in the stable maintenance of plasmid pMF1 in M. fulvus cells using a post-segregational killing mechanism

RESULTS

DISCUSSION

EXPERIMENTAL PROCEDURES