Analysis of the CtrA Pathway in Magnetospirillum has revealed an ancestral role in motility in the Alphaproteobacteria.

What is its role to magnetotactic bacteria?

Bacteria use a variety of mechanisms to inhabit a large diversity of environments. One of these organisms is the Magnetotactic Bacteria (MB) which has the ability to navigate along the earth’s magnetic field to find hypoxic environments. This is due to the presence of an organelle called the magnetosome. Magnetosome’s are held together in chains in sub cellular compartments, providing the cell with the capability to align in magnetic fields.

Regulation of developmental events across the cell cycle in Alphaproteobacteria has been investigated most thoroughly on Caulobacter crescentus. The execution of cell division requires the timed coordination of key events, this requires a regulatory network. At the core of this network is the response regulator CtrA. In C. crescentus CtrA is involved in regulation of gene expression and many more key processes. CtrA has been shown to have various roles in the cell division cycles of other bacteria in the phylum. The diverse roles of CtrA in species of Alphaproteobacteria has raised questions as most components have been proven to be important but yet play vastly different roles in the cells of different species.

In this study Greene et al 2012 looked at the role of the CtrA regulatory network and how sub cellular compartments are effected during cell division, which is conserved throughout the Alphaproteobacteria, on magnetotactic bacterium Magnetospirillum magneticum strain AMB-1.

Genetic analysis of this species has identified a genomic region that is essential for magnetosome formation which is now termed the magnetosome island (MAI). Whilst progress has been made into uncovering genes that are essential in the steps of magnetosome formation, the integration of these processes is poorly understood.

Greene et al 2012 took a targeted approach to investigate the regulation of CtrA in the context of a microorganism possessing intracellular organelles by creating Mutants of the AMB-1 with deletions to CtrA and DivK, CtrA’s negative regulator. They found that these control genes were not essential for keeping the cell viable, but the mutants had motility defects indicating that control genes play an important role in cell motility. A ctrA deletion in a highly motile MIA deletion strain stopped motility, whilst a deletion of divK induced motility in a previously nonmotile wild-type of AMB-1 (WT).  

This study also found that CtrA’s activity in AMB-1 is controlled in a manner similar to that of C.crescentus. Greene and colleagues results suggest that the phosphorylation state of CtrA is essential for motility in AMB-1 and that swimming behaviour observed in the divK deletion strain potentially comes from an increased phosphorylated and active CtrA.

Most species of magnetotactic bacteria swim constitutively when grown in culture. Interestingly, in this study, Greene et al found that the vast majority of wild-type AMB-1 cells in culture were non motile. This is in stark contrast to other published studies in which entire populations of AMB-1 cells were found to be swimming. It is possible that differences in growth mediums or environmental conditions caused the differences seen in past studies. However, it is also likely that motile cells were selected for using a magnetically guided swimming-based procedure termed the “racetrack” assay. This is a common practice among some groups working with magnetotactic bacteria. In the case of AMB-1, such a procedure might select for genetic variants that would give rise to constitutively swimming strains. Given Greene et al’s hypothesis explaining the evolutionary benefits of magnetosome chain formation, the lack of motility in AMB-1 was surprising. Wild-type AMB-1 failed to swim even with increased or decreased iron or oxygen concentrations. Another potential reason could be that the isolation and current growth conditions were enough to satisfy the nutritional and energetic requirements of AMB-1 in this experiment, thus negating any reason to devote energy toward movement.

The common feature of the CtrA pathways in many alphaproteobacteria is its role in motility and cell division. Genetic analyses in C. crescentus, and now M. magneticum AMB-1 have shown the involvement of CtrA in the regulation of motility. Greene and co conducted an analysis of the phylogenetic relationships among CtrA alleles from organisms in which its biological role has been investigated and suggested that regulation of motility by CtrA is an ancestral trait. In light of the studies findings, Greene et al put forward that the ancestral alphaproteobacterium was motile and during evolution, acquired regulation of motility by the response regulator CtrA. The results of the study presented here are the most comprehensive experimental examination of a CtrA regulon in an organism. They have provided a glimpse into the evolution and divergent specialisation of this important regulator and provide the basis for future detailed mechanistic studies into its function in AMB-1.

I have decided to review this paper because I have a keen interest in MB and wanted to gain a better understanding of how these organisms work. I also wanted to increase awareness of MB. I feel this paper makes a good contribution to the field but given the long history of this area, i don’t think my review sufficiently cover’s its scope. I suggest that readers interested in gaining a better understanding of the research into MB should go and do some more digging.

S.E. Greene. M. Brilli. B Emanuele. E. Biondi, and A. Komeilia (2012) Analysis of the CtrA Pathway in Magnetospirillum Reveals an ancestral Role in Motility in Alphaproteobacteria. Journal of bacteriology June 2012 Volume 194 Number 11:. 2973–2986

http://jb.asm.org/content/194/11/2973.full.pdf+html