Abstract
This Thesis presents evidence to show how the accumulation and decline of the cytochrome P-450IA1 within the cells of the yeast Saccharomyces cerevisiae being grown in a 14% glucose medium is intimately associated not only with the actual Dissolved Oxygen Tension of the medium but corresponds to the changing levels of oxygen in the medium. Emphasis is placed on the finding that the history of the treatment of the culture is important in determining the present response of the cells to the change in the Dissolved Oxygen Tension of the medium. Evidence is presented to support the finding that, given certain pre-requisits in the history of the fermentation and also that the whole fermentation is conducted within a certain band of dissolved oxygen tension, the rise in cytochrome P-450IA1 content of the yeast cells is associated with a time of increasing level of dissolved oxygen in the medium. Similarly, the loss of cytochrome P-450IA1 from the yeast cells is associated with falling levels of oxygen in the medium. Of particular importance is the process of recovery from Anoxia where the yeast culture has previously been grown anaerobically and is then supplied with oxygen by turning on the air supply. Brief mention is made of the potential commercial importance of repeatedly subjecting a culture to anaerobic and aerobic conditions. Several other fermentation factors such as the ethanol concentration of the medium, the demand for alkali, and the percentage of carbon dioxide in the exhaust gases were also examined in detail to determine their value in predicting the amount and timing of cytochrome P-450IA1 produced by the culture. The evidence presented is drawn from the numerous fermentation experiments carried out by the author over the eight years from January 1983 to the present. Vital to these investigations was the use of a computer to control precisely the fermentation parameters such as pH, agitation rate, and temperature as well as recording the measurements made by probes inserted into the fermentation tank. Although the vast majority of the data is drawn from experiments carried out in a computer-controlled 5 litre fermentation tank, comparisions are also made with fermentations carried out in 250 cm3 shake flasks. Great reliance was placed throughout the work on the ability to draw numerous samples of cells and medium from the same fermentation, so as to accurately track the events in that fermentation. Not only were many sample required but these needed to be drawn at regular intervals throughout the 24 hour period to ensure that no events were missed during the night. The raw data from the experiments which was collected at the time is here presented in the form of graphs which best illustrate the findings of the research. Although this research work was mostly carried out using the yeast strain Saccharomyces cerevisiae NCYC 754, mention is also made of the occurence of cytochrome P-450IA1 in other strains.