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The unicellular alga Dunaliella tertiolecta was cultivated axenically with histidine, or ammonium, or histidine and ammonium ions as nitrogen sources. In the presence of histidine as the sole source of nitrogen, cell growth is comparable to that observed with the same concentration of nitrogen in ammonium form. In the presence of both histidine and ammonium, histidine degradation is observed only when the concentration of ammonium falls below 4 mM. Under these conditions, the first two enzymes of histidine degradation pathway, histidase and urocanase, as well as histidine permease, are produced. Linear relationships are observed between histidase and urocanase and histidase and histidine permease, respectively, showing that, in D. tertiolecta enzyme, systems are coordinately regulated. Histidase activity is also controlled by succinate, glutamate and acetate as carbon sources. D. tertiolecta is unable to grow on a medium containing acetate and histidine together in the absence of ammonium. |
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