Optimal conditions for induction of competence in nitrogen-fixing Azotobacter vinelandii

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Abstract
Competence development in N-fixing A. vinelandii cells was optimal at pH 7.2-7.4, which necessitated additional buffering of the Fe-limited N-free competence medium or the addition of a suitable organic acid salt, e.g., sodium acetate. An autolysin was active in this pH range, and competent cells were more susceptible to autolysis than the general cell population. Competence development also required restricted aeration of the culture and only those cultures that attained zero dissolved O2 became competent. Restricted aeration served to protect the Fe-limited cell nitrogenase from O2 inactivation, allowing the culture to reach zero dissolved O2. The inclusion of additional sources of reductant, e.g., malate, in buffered competence medium resulted in increased respiration and protection of nitrogenase, increased cell mass and poly-.beta.-hydroxybutyrate synthesis, but decreased competence. A possible explanation for the apparent competition between competence development and nitrogenase activity is discussed.
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