Regulation of Nitrogen Assimilation by the Obligate Chemolithotroph Thiobacillus neapolitanus

Abstract

The obligate chemolithotroph Thiobacillus neapolitanus can grow with NH4 + NO3 − or urea as source of nitrogen. Gradual and rapid mechanisms were detected for regulating both the activity and the rate of synthesis of enzymes required for the metabolism of these nitrogen compounds. Glutamine synthetase (GS) in combination with glutamate synthase (GOGAT) was active under most growth conditions. Alanine dehydrogenase appears to be the major pathway of NH4 + assimilation during energy-limited growth in the presence of excess NH4 +. GS was regulated in this organism by repression/derepression of enzyme synthesis, by inhibition by low molecular weight compounds, and also by adenylylation and deadenylylation. GS was deadenylylated during CO2- and N-limited growth and also during energy-limited growth when NO3 − or urea were supplied as the nitrogen source. GS was adenylylated during energy-limited growth in the presence of 7·7 mm-NH4 +. The activity of GS increased with decreasing dilution rate during NH4 +-limited growth, whereas the activity of GOGAT remained almost constant. The ability of whole cells to reduce NO3 − was derepressed during N-limited growth. During NH4 +-limited growth 24 % of total carbon fixed was excreted as 2-oxoglutarate, pyruvate, succinate, p-hydroxyphenylacetate and ethylmalonate.