Regulation of<scp>l</scp>-Alanine Dehydrogenase in<i>Rhizobium leguminosarum</i>bv.<i>viciae</i>and Its Role in Pea Nodules

DOI Web Site 被引用文献1件
  • Emma Lodwig
    Division of Microbiology, School of Animal and Microbial Sciences, University of Reading, Whiteknights, Reading, RG6 6AJ, United Kingdom
  • Shalini Kumar
    Division of Microbiology, School of Animal and Microbial Sciences, University of Reading, Whiteknights, Reading, RG6 6AJ, United Kingdom
  • David Allaway
    Division of Microbiology, School of Animal and Microbial Sciences, University of Reading, Whiteknights, Reading, RG6 6AJ, United Kingdom
  • Alex Bourdes
    Division of Microbiology, School of Animal and Microbial Sciences, University of Reading, Whiteknights, Reading, RG6 6AJ, United Kingdom
  • Jürgen Prell
    Ökologie des Bodens, Botanisches Institut, RWTH-Aachen, 52056 Aachen, Germany
  • Ursula Priefer
    Ökologie des Bodens, Botanisches Institut, RWTH-Aachen, 52056 Aachen, Germany
  • Philip Poole
    Division of Microbiology, School of Animal and Microbial Sciences, University of Reading, Whiteknights, Reading, RG6 6AJ, United Kingdom

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<jats:title>ABSTRACT</jats:title><jats:p>Alanine dehydrogenase (AldA) is the principal enzyme with which pea bacteroids synthesize alanine de novo. In free-living culture, AldA activity is induced by carboxylic acids (succinate, malate, and pyruvate), although the best inducer is alanine. Measurement of the intracellular concentration of alanine showed that AldA contributes to net alanine synthesis in laboratory cultures. Divergently transcribed from<jats:italic>aldA</jats:italic>is an AsnC type regulator,<jats:italic>aldR.</jats:italic>Mutation of<jats:italic>aldR</jats:italic>prevents induction of AldA activity. Plasmid-borne<jats:italic>gusA</jats:italic>fusions showed that<jats:italic>aldR</jats:italic>is required for transcription of both<jats:italic>aldA</jats:italic>and<jats:italic>aldR</jats:italic>; hence, AldR is autoregulatory. However, plasmid fusions containing the<jats:italic>aldA-aldR</jats:italic>intergenic region could apparently titrate out AldR, sometimes resulting in a complete loss of AldA enzyme activity. Therefore, integrated<jats:italic>aldR</jats:italic>::<jats:italic>gusA</jats:italic>and<jats:italic>aldA</jats:italic>::<jats:italic>gusA</jats:italic>fusions, as well as Northern blotting, were used to confirm the induction of<jats:italic>aldA</jats:italic>activity. Both<jats:italic>aldA</jats:italic>and<jats:italic>aldR</jats:italic>were expressed in the II/III interzone and zone III of pea nodules. Overexpression of<jats:italic>aldA</jats:italic>in bacteroids did not alter the ability of pea plants to fix nitrogen, as measured by acetylene reduction, but caused a large reduction in the size and dry weight of plants. This suggests that overexpression of<jats:italic>aldA</jats:italic>impairs the ability of bacteroids to donate fixed nitrogen that the plant can productively assimilate. We propose that the role of AldA may be to balance the alanine level for optimal functioning of bacteroid metabolism rather than to synthesize alanine as the sole product of N<jats:sub>2</jats:sub>reduction.</jats:p>

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