In order to gain a clear idea of how plasmids or bacteria function on their own, it is necessary to have some understanding of how the two interact. There are, for example, obvious ways in which one influences the other, such as the conference of useful phenotypic traits (e.g. antibiotic resistance) to the host and the molecular maintenance (e.g. DNA replication) of plasmids by the host. There are also much more subtle interactions between the two, which are just now being elucidated for the first time. One such interaction is described by Kolatka et. al. in their recent publication.
This article describes the interactions of the broad-host-range IncP-1 plasmid RK2 and the partitioning systems of Pseudomonas putida and Escherichia coli. The authors found that the subcellular location of a given type of plasmid in a given strain of host will have a particular location within the cell. Also, and even more interestingly, this position depends on the protein products of both the host and plasmid.
Comparing the subcellular location of an RK2 mini-derivative in E. coli and P. putida showed this interaction between host and plasmid. Because the plasmid lacked an active partitioning system, the partitioning machinery of its host determined its position. In the case of E. coli, the plasmid was found to cluster at the cell poles, whereas in P. putida it was located either at a mid-cell or one quarter of the way into the cell on either side. The location within P. putida was explained by the interactions between the centromere-like sequences on the plasmid and the ParB protein encoded by the host’s chromosome. Indeed, when the parB gene was inserted into the E. coli chromosome the resultant partitioning mirrored that of P. putida. Conversely, when the par genes in P. putida where made nonfunctional then the plasmids were found at the poles. The position of RK2 itself showed similar locational disruptions when its position was determined in the P. putida par mutants. In all cases, plasmid location was determined by fluorescence in situ hybridization (FISH) and fluorescence microscopy. Protein interactions between the plasmids and bacteria were determined by formaldehyde cross-linking and chromatin immunoprecipitation.
It is shown here that, as always, the actions and activity of an individual organism (or even a mobile genetic element) are by no means completely independent, but rather that they are constantly influenced by the organisms and environments that they come into contact with. Indeed, the basic tenets of evolution involve, not isolated individuals, but the interaction between individuals. Therefore, in the case of bacteria and plasmids, it is necessary to not only study one and then the other, but also the influence that they have on one another.
Kolatka, K., M. Witosinska, M. Pierechod, and I. Konieczny. 2008. Bacterial partitioning proteins affect the subcellular location of broad-host-range plasmid RK2. Microbiology. 154:2847-56.
Ebersbach, G. & Gerdes, K. (2005). Plasmid segregation mechanisms. Annu Rev Genet 39, 453–479.
Funnell, B. E. (2005). Partition-mediated plasmid pairing. Plasmid 53, 119–125.
Gordon, S., Rech, J., Lane, D. & Wright, A. (2004). Kinetics of plasmid segregation in Escherichia coli. Mol Microbiol 51, 461–469.
Department of Biological Sciences
University of Idaho