Control of Isocitratase Formation in Rhizopus Nigricans

Wegener, Warner S. (University of Cincinnati, Cincinnati, Ohio), and Antonio H. Romano. Control of isocitratase formation in Rhizopus nigricans. J. Bacteriol. 87:156-161. 1964.-A fumaric acid-producing strain of Rhizopus nigricans was found to produce a fair level of isocitratase in a casein hydrolysate medium. Glucose repressed enzyme formation. When glucose was utilized during growth, there was a relief of repression, and enzyme synthesis was resumed at a rate equivalent to that found in nonrepressed cells. Zinc stimulated isocitratase formation in glucose-repressed cultures by stimulating growth and glucose utilization, thereby decreasing accumulation of repressor metabolites derived from glucose. The effectiveness of acetate as an inducer was greater on glucose-repressed cells than on nonrepressed cells; cells grown in the presence of glucose formed higher levels of isocitratase when subsequently replaced with an acetate-containing inductive medium than did cells grown without glucose. Moreover, addition of 2 ppm of Zn(++) during the inductive replacement phase resulted in a twofold increase in isocitratase formation. The hypothesis is submitted that Zn(++) exerts its action by stimulating ribonucleic acid (RNA) synthesis, thereby facilitating the formation of a specific RNA during induction. Preliminary evidence implicating Zn(++) in the stimulation of RNA synthesis in this organism is presented.