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Effect of an ntrC mutation on amino acid or urea utilization and on nitrogenase switch-off in Herbaspirillum seropedicae

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Herbaspirillum seropedicae is a nitrogen-fixing bacterium that grows well with ammonium chloride or sodium nitrate as alternative single nitrogen sources but that grows more slowly with -alanine, -serine, -proline, or urea. The ntrC mutant strain DCP286A was able to utilize only ammonium or urea of these nitrogen sources. The addition of 1mmol·L–1 ammonium chloride to the nitrogen-fixing wild-type strain inhibited nitrogenase activity rapidly and completely. Urea was a less effective inhibitor; approximately 20% of nitrogenase activity remained 40min after the addition of 1mmol·L–1 urea. The effect of the ntrC mutation on nitrogenase inhibition (switch-off) was studied in strain DCP286A containing the constitutively expressed gene nifA of H. seropedicae. In this strain, nitrogenase inhibition by ammonium was completely abolished, but the addition of urea produced a reduction in nitrogenase activity similar to that of the wild-type strain. The results suggest that the NtrC protein is required for assimilation of nitrate and the tested amino acids by H. seropedicae. Furthermore, NtrC is also necessary for ammonium-induced switch-off of nitrogenase but is not involved in the mechanism of nitrogenase switch-off by urea.

Herbaspirillum seropedicae est une bactérie fixatrice d’azote qui pousse bien lorsque le chlorure d’ammonium ou le nitrate de sodium sont utilisés comme source alternative d’azote, mais croissent plus lentement sur la -alanine, la -proline ou l’urée. La souche DCP286A dont le gène ntrC est muté était capable d’utiliser l’ammonium ou l’urée comme seules sources d’azote. L’ajout de chlorure d’ammonium 1mmol·L–1 à la souche fixatrice d’azote sauvage inhibait l’activité de la nitrogénase rapidement et complètement. L’urée était un inhibiteur moins efficace: approximativement 20% de l’activité de la nitrogénase persistait, 40 min après l’ajout de 1mmol·L–1 d’urée. L’effet de la mutation sur ntrC sur l’inhibition de la nitrogénase (switch-off) a été étudié chez la souche DCP286A exprimant de façon constitutive nifA de H.seropedicae. Dans cette souche, l’inhibition de la nitrogénase par l’ammonium était complètement abolie, mais l’ajout d’urée produisait une réduction de l’activité de la nitrogénase équivalente à celle observée dans la souche sauvage. Ces résultats suggèrent que la protéine NtrC est requise pour l’assimilation du nitrate et les acides aminés testés chez H.seropedicae. Qui plus est, NtrC est aussi nécessaire à l’inhibition de la nitrogénase induite par l’ammonium, mais n’est pas impliquée dans le mécanisme d’inhibition de la nitrogénase par l’urée.

Document Type: Research Article

Publication date: March 3, 2008

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  • Published since 1954, this monthly journal contains new research in the field of microbiology including applied microbiology and biotechnology; microbial structure and function; fungi and other eucaryotic protists; infection and immunity; microbial ecology; physiology, metabolism and enzymology; and virology, genetics, and molecular biology. It also publishes review articles and notes on an occasional basis, contributed by recognized scientists worldwide.
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