| Abstract: | To obtain information on the route(s) of ammonium assimilation in Streptomyces venezuelae, cell suspensions transferred to fresh medium lacking nitrogen were pulsed with 15N2]ammonium sulphate. Cells and extracellular fluids were examined by nuclear magnetic resonance and amino acid analysis to assess changes in amino acid pools and the disposition of 15N]ammonium. Following addition of 15N]ammonium, glutamate--glutamine pools of low cell density replacement cultures expanded rapidly and became progressively labelled with 15N, whereas the alanine pool size increased much more slowly and became labelled with 15N to a much lesser extent. These results are consistent with the assimilation of ammonium via glutamate dehydrogenase or glutamine synthetase--glutamate synthase rather than alanine dehydrogenase. Under anaerobic conditions, S. venezuelae assimilates ammonium into alanine rather than glutamate--glutamine. Alanine dehydrogenase may thus function as a vehicle to regenerate NAD+ to maintain substrate-level phosphorylation during periods of anaerobiosis. |
