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Rhizobium meliloti carries two megaplasmids
Abstract:
In Rhizobium meliloti strain 41 the existence of a second megaplasmid (pRme41c) with a molecular weight similar to the sym megaplasmid pRme41b was demonstrated. Derivatives of the wild-type strain carrying pRme41b or pRme41c tagged with Tn5 allowed the examination of the transfer ability of both megaplasmids. The introduction of megaplasmids into the wild-type R. meliloti was not detected, probably because of the action of plasmid genes coding for entry exclusion of the same type of plasmid. However, transmissibility of both megaplasmids was observed in matings with Nod- or Fix- pRme41b deletion mutant recipients and with Agrobacterium tumefaciens at frequencies of 10(-6) - 10(-8). Introduction of the megaplasmids into the R. meliloti recipients resulted in the loss of the same plasmid. On the other hand, pRme41b and pRme41c were compatible. From the extent of deletions in various Nod- and Fix- mutants a DNA region carrying genes probably involved in "surface exclusion" on pRme41b was located. This DNA region is about 50 kb distant from the nod genes and exhibits strong homology with a DNA segment of pRme41c. Symbiotic genes on pRme41c were not identified.
Insights
Rhizobium meliloti strain 41 harbors two megaplasmids, pRme41b and pRme41c. Researchers found these megaplasmids can transfer to other bacteria, with pRme41b containing genes for surface exclusion.
Area of Science:
- Microbiology
- Genetics
- Molecular Biology
Background:
- Rhizobium meliloti strain 41 is known to possess symbiotic megaplasmids.
- The existence and characteristics of a second megaplasmid, pRme41c, were investigated.
Purpose of the Study:
- To demonstrate the existence of a second megaplasmid, pRme41c, in R. meliloti strain 41.
- To examine the transfer ability and compatibility of megaplasmids pRme41b and pRme41c.
- To locate genes involved in surface exclusion on megaplasmid pRme41b.
Main Methods:
- Transposon (Tn5) tagging of megaplasmids pRme41b and pRme41c.
- Bacterial conjugation experiments with various R. meliloti mutants and Agrobacterium tumefaciens recipients.
- Analysis of plasmid deletions in Nod- and Fix- mutants.
Main Results:
- A second megaplasmid, pRme41c, with similar molecular weight to pRme41b was identified.
- Both pRme41b and pRme41c exhibited transmissibility to R. meliloti and A. tumefaciens recipients.
- Entry exclusion mechanisms likely prevent megaplasmid transfer into wild-type R. meliloti.
- A DNA region on pRme41b, approximately 50 kb from nod genes, was implicated in surface exclusion and showed homology to pRme41c.
- Symbiotic genes on pRme41c were not identified.
Conclusions:
- R. meliloti strain 41 possesses at least two transferable megaplasmids, pRme41b and pRme41c.
- Megaplasmid transfer is regulated by surface exclusion mechanisms.
- A distinct DNA region on pRme41b likely controls surface exclusion and shares homology with pRme41c.