Positions
Energisation of Nitrogen
Fixation in the Rhizobium-
legume symbiosis
Post Doctoral Fellowship Available (May 2009)
This BBSRC funded position is available from April 2008 and has been advertised in www.jobs.ac.uk from where formal details of application can be found.
Nitrogen fixation within the Rhizobium-legume symbiosis is the main driver of the global nitrogen cycle. The process is driven by a supply of carbon from the plant. Therefore, by understanding this we can determine which factors regulate the total amount of nitrogen fixed. The regulation of nutrient exchange between a legume and Rhizobium is largely determined by two membrane systems, one is plant derived and the other is bacterial. Our approach to understanding the interactions within and between these membranes is to mutate and clone the bacterial transport systems important for nutrient exchange. The genetics governing regulation of expression of these transport systems is being investigated and their effects on nitrogen fixation assessed. We have shown that the plant must supply amino acids to the bacteroid to sustain nitrogen fixation (Lodwig et al 2002; Prell and Poole 2006). Our aim is to understand why the plant must supply amino acids to the root nodule bacteria and how this controls the development of symbiosis. In this project we aim to determine how amino acid metabolism and cycling between the plant and bacteria provides the energy for nitrogen fixation. Genetical and biochemical approaches will be used to examine the regulation of flux of amino acids through the bacteroid. The post holder will examine how the main amino acid permeases (Aap and Bra) of R. leguminosarum are regulated and how this alters nodule metabolism. For further information about this post contact Philip Poole (phone 01603 450750).Lodwig, E.M., Hosie, A.H.F., Bourdes, A., Findlay, K.,
Karunakaran, R., Downie, J.A. and Poole P.S. (2003) Amino-acid cycling drives
nitrogen fixation in the legume-Rhizobium symbiosis. Nature 422: 722-726.
(pdf)
Jurgen Prell and Philip Poole (2006) Metabolic changes in rhizobia. Trends in
Microbiology. 141: 161-168. (pdf)
The post holder will work closely with a team of Postdoctoral Research Assistants working in the laboratory of Philip Poole.
The Poole group works in the area of plant microbe interactions with particular emphasis on nitrogen fixation in the Rhizobium legume symbiosis. A wide variety of approaches are used including genetics, molecular biology, enzymology, bacterial physiology, plant physiology and micro array analysis. There is considerable expertise in these areas within the current group so training in new techniques will be provided.
Transcriptomic analysis of
bacteroid formation
(Open trawl Ph.D. students)
Rhizobium
leguminosarum
The two plants on the left show the
bacteroids in bean nodules
effect of a metabolic block that
prevents amino acid uptake
by R. leguminosarum
Transcriptomic analysis of the transition from a
free-living cell to a root nodule bacteroid in
R. leguminosarum.
The sequencing of the genome of R.
leguminosarum offers us a unique opportunity to use microarrays to address
the problems both
of the final metabolic state of the bacteroid and the genetic and
developmental switches that are crucial to differentiation. By comparing
gene expression between free-living bacteria, rhizosphere bacteria and
bacteria/bacteroids from various stages of nodule development
we can begin to reconstruct the regulatory switches and developmental/
metabolic consequences of these switches.
In this project I propose to investigate two
aspects of bacteroid development and metabolism and a general aspect of
environmental
gene regulation, which are:
1) How does bacteroid metabolism differ from a free-living cell and how is this important to nitrogen fixation,
2) What are the genetic and development changes that occur during the transition of free-living bacteria to bacteroids.
Specifically I hypothesise that
the three dominant factors regulating bacteroid metabolism are the O2
tension, provision of
C4-dicarboxylates and cellular growth rate, therefore gene
expression profiles of rhizosphere bacteria and bacteroids will be compared
against free-living bacteria grown in chemostat culture limited by either
glucose, malate or O2 at intermediate and low dilution rates.
The expression data will then be mapped to the KEGG metabolic database using
the GeneSpring bionformatic analysis suite to allow
us to visualise the key metabolic changes that occur in bacteroid formation.
With regard to the second aim
above we will analyse the genetical and developmental changes that occur
during the transition
of free-living bacteria to bacteroids. This will be tackled by comparing the
gene expression profiles of free-living bacteria, fully
developed bacteroids and bacteroids arrested in development by well-defined
mutations. These mutants are lpcB, which codes for
CMP-Kdo:LPS Kdo transferase which is needed for formation of the LPS core
, dctA, which codes for the dicarboxylate transport
system (bacteroids are fully formed but cannot fix nitrogen), bacA, the
role of which is not understood, but is needed for development
of bacteroids and a double amino acid transport mutant (aap/bra) which
forms fully developed bacteroids capable of reducing N2 to
NH3 but which still forms ineffective nodules. By comparing gene
expression in cells blocked at various developmental stages I
believe we can determine what are the key changes in the transition from a
free-living cell to a bacteroid.
References (hyperlinked papers can be downloaded)
2.Poole, P.S. and Allaway, D. (2000) Carbon and nitrogen metabolism in Rhizobium. Advances in Microbial Physiology 43:117-163.
3. Arthur Hosie and Philip Poole (2001) Bacterial ABC transporters of amino acids. Current Microbiology 152:259-270.
For further information contact Philip Poole. This page was last updated on 08-Apr-2009.