Regulation der UDPG-Pyrophosphorylase-Aktivität in Acetabularia

Abstract

The activity of UDPG-pyrophosphorylase [EC 2.7.7.9] was investigated during the morphogenesis (stalk and cap formation) of the unicellular and uninucleate green alga Acetabularia mediterranea. The activity of this enzyme is strongly related to the development of the cap. There was a substantial increase in the activity of the enzyme during cap formation. This increase in enzyme activity during cap formation also took place in anucleate cells. These findings are in good agreement with the fact that the glucose and galactose content of the cell wall of the cap is much higher than that of the stalk wall. The increase in enzyme activity in nucleate and anucleate cells was reversibly stopped by puromycin and actidion. Therefore the increase in the pyrophosphorylase activity must be due to synthesis of this enzyme. Actinomycin inhibits only the synthesis of the pyrophosphorylase in regenerating nucleate posterior parts of the cell. No inhibition occurs in anucleate cells with intact apical stalk because the cells contain a store of relatively stable messenger-RNA in the anterior parts of the stalk. On the other hand actinomycin inhibits the increase in chlorophyll content in both cell parts nucleate or anucleate. These findings indicate that the synthesis of UDPG-pyrophosphorylase in Acetabularia does not involve a direct regulatory function of nuclear or extranuclear genes (as for example chloroplast genes). Therefore we have to conclude that the synthesis of this enzyme was regulated in the cytoplasm at the level of translation.