Regulation of macromolecular synthesis during nutritional shift-up in the fungus Mucor

The dimorphic fungus Mucor racemosus was grown anaerobically in the yeast form and shifted from a defined minimal medium ( mu = 0.17 doublings/h) to an enriched complex medium ( mu = 0.43 doublings/h). The measured rates of increase of several growth-related parameters displayed an immediate and complete adjustment to the higher growth rate without an intervening lag. Pulse-labeling experiments indicated that the rates of protein and RNA synthesis increased immediately and in parallel in response to the shift. The rate of polypeptide chain elongation immediately increased by approximately 60% to support the new higher rate of protein accumulation, but later declined to near pre-shift values. The accelerated rate of protein accretion was apparently sustained by a gradual increase in the total number of ribosomes per weight of cellular material and by a more rapid increase in the percentage of ribosomes recruited into polysomes at a given time.