Abstract
pMF1 is the only autonomously replicating plasmid that has been recently identified in myxobacteria. This study characterized the partitioning (par) system of this plasmid. The fragment that significantly increased the retaining stability of plasmids in Myxococcus cells in the absence of selective antibiotics contained three open reading frames (ORFs) pMF1.21-pMF1.23 (parCAB). The pMF1.22 ORF (parA) is homologous to members of the parA ATPase family, with the highest similarity (56%) to the Sphingobium japonicum ParA-like protein, while the other two ORFs had no homologs in GenBank. DNase I footprinting and electrophoretic mobility shift assays showed that the pMF1.23 (parB) product is a DNA-binding protein of iteron DNA sequences, while the product of pMF1.21 (parC) has no binding activity but is able to enhance the DNA-binding activity of ParB to iterons. The ParB protein autogenously repressed the expression of the par genes, consistent with the type Ib par pattern, while the ParC protein has less repressive activity. The ParB-binding iteron sequences are distributed not only near the partitioning gene loci but also along pMF1. These results indicate that the pMF1 par system has novel structural and functional characteristics.
Highlights
Myxobacteria are well known for their unique multicellular social behavior [1] and the production of diverse and novel bioactive secondary metabolites [2]
In 2008, we reported the discovery of the first and as yet only endogenous myxobacterial plasmid, pMF1, which was isolated from Myxococcus fulvus strain 124B02 [3]
We localized the replication region to the pMF1.14 open reading frames (ORFs) using a vector that cannot replicate in M. xanthus, and shuttle vectors between Escherichia coli and M. xanthus were successfully constructed [3]
Summary
Myxobacteria are well known for their unique multicellular social behavior [1] and the production of diverse and novel bioactive secondary metabolites [2]. Except for a few ORFs that are highly homologous to those in myxobacterial genomes, most ORFs in the Myxococcus plasmid pMF1 do not have homologs in the GenBank database, including the replication-associated sequence. We localized the replication region to the pMF1.14 ORF using a vector that cannot replicate in M. xanthus, and shuttle vectors between Escherichia coli and M. xanthus were successfully constructed [3]. Because they lack a stabilizing sequence, these low-copy-number shuttle vectors are not stably inherited in M. xanthus and are lost in the absence of selective antibiotics
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