The effect of agitation on the control of secondary metabolism in liquid cultures of penicillium expansum
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The effect of agitation on secondary metabolism in Penicillium expansum has been investigated by studying primary metabolic enzyme activities, mycelial lipid accumulation, and the production of the polyketides patulin and citrinin. The results presented here clearly show that agitation directed the flux of acetyl CoA into lipid accumulation rather than polyketide biosynthesis, particularly in the malt extract medium used in these studies. This would be a result of citrate accumulation inducing fatty acid synthesis through the increased activity of the Tricarboxylic Acid (TCA) cycle through increased O2 concentrations. High glucose concentrations in the Yeast Extract Glucose Buffer (YEGB) medium would result in catabolite repression of this activity, and O2 diffusion to the mycelium would have been reduced through the thick mycelial suspensions that formed in the shake cultures. The supply of acetyl CoA to polyketide synthesis, controlled through the availability of O2 to respiratory processes, is considered a major regulatory factor in this organism. The availability of reducing power in NADPH was also a significant factor favouring lipid synthesis over polyketides in shake cultures. The pentose phosphate pathway was more active in shake cultures than static, as were the other potential sources of NADPH in the cytoplasm - malic enzyme and the NADP+ -utilising form of isocitrate dehydrogenase. The ammonium concentration was not likely to repress polyketide synthesis in either medium. There was some evidence to suggest that patulin synthesis was affected by feedback inhibition of late pathway metabolites or by intracellular protease activity. Finally, the presence of O2 also affected the route taken by intermediates of patulin biosynthesis. The presence of toluquinol in the shake culture filtrates was possibly due to O2 activation of a mixed function oxidase competing for m-cresol with another producing m-hydroxybenzyl alcohol, normally present in static cultures.