Regulation of ornithine decarboxylase during morphogenesis of Mucor racemosus.

During the yeast-to-hyphae transition of the dimorphic phycomycete Mucor racemosus, there was a 30- to 50-fold increase in the activity of ornithine decarboxylase. Increased enzyme activity preceded the emergence of germ tubes and reached a maximum before conversion was completed. Subsequently, enzyme levels rapidly declined, despite the continuation of mycelial growth. Both putrescine and spermidine blocked the enzyme activity response. Protein synthesis was required for the increase in enzyme activity during morphogenesis. A combination of actinomycin D and netropsin inhibited ribonucleic acid synthesis but failed to inhibit the increase in ornithine decarboxylase activity. There was a twofold increase in the enzyme half-life during morphogenesis with either trichodermin or verrucarin to inhibit protein synthesis.