Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Cells of the dimorphic Zygomycetous fungus Mucor racemosus were grown in both batch and continuous culture in order to study the regulation of RNA and protein synthesis as a function of (i) growth rate, (ii) morphogenesis and (iii) nutritional shift-up. In all three cases cells were analyzed for parameters used to calculate (i) cellular ribosome concentration, (ii) percentage of ribosomes active in protein synthesis and (iii) polypeptide chain elongation rate. Cultures subjected to atmospheric or nutritional shifts were additionally pulse labelled or pulse labelled and steady-state labelled with radioactive amino acids and phosphate to measure the rates of synthesis and accretion of proteins and nucleic acids respectively. In continuous culture studies, the polypeptide chain elongation rate and the percentage of ribosomes active in translation were both linear functions of the growth rate. In continuous culture under a nitrogen atmosphere, cellular ribosome concentration was also a linear function of growth rate. However, in continuous culture under a carbon dioxide atmosphere, this parameter remained relatively constant regardless of growth rate. During atmospheric shifts, an increase in polypeptide chain elongation rate was an invariant correlate of an increase in growth rate. However, no consistent correlation was observed between this parameter and morphogenesis of the cells. In nutritional shifts-up, a post-shift lag of one hour was observed before total cellular protein/ml of culture, cellular dry weight/ml of culture and culture turbidity achieved new, higher rates of increase. This was corroborated by results from steady-state labelling experiments using L-{('14)C}leucine and {('32)P}phosphate. Pulse labelling with the same isotopes revealed the post-shift increase in the rate of RNA synthesis lagged thirty minutes behind the corresponding increase in the rate of protein synthesis. The fungus also responded to shift-up conditions with a rapid and sustained increase in the polypeptide chain elongation rate and in the percentage of ribosomes active in protein synthesis. The foregoing observations are evidence for the existence in Mucor, and perhaps in other eukaryotes, of a system for regulating the rate of protein synthesis which is adjustable at the levels of ribosome recruitment, ribosome velocity as it traverses the message, and possibly cellular ribosome concentration as well.