HISTIDINE UTILIZATION BY THE UNICELLULAR ALGA DUNALIELLA-TERTIOLECTA

The unicellular alga Dunaliella tertiolecta was cultivated axenically with histidine, or ammonium, histidine and ammonium ions as nitrogen s ources. In the presence of histidine as the sole source of nitrogen, c ell growth is comparable to that observed with the same concentration of nitrogen in ammonium form. In the presence of both histidine and am monium, histidine degradation is observed only when the concentration of ammonium falls below 4 mM. Under these conditions, the first two en zymes of histidine degradation pathway, histidase and urocanase, as we ll as histidine permease, are produced. Linear relationships are obser ved between histidase and urocanase and histidase and histidine permea se, respectively, showing that, in D. tertiolecta enzyme, systems are coordinately regulated. Histidase activity is also controlled by succi nate, glutamate and acetate as carbon sources. D. tertiolecta is unabl e to grow on a medium containing acetate and histidine together in the absence of ammonium. (C) 1998 Elsevier Science Inc.