The relationship of growth rate and catabolite repression with WHI2 expression and cell size in Saccharomyces cerevisiae

Mixtures of D-glucosamine and glucose were used to slow the growth of wild-type and whi2 mutant strains of Saccharomyces cerevisiae without affecting the level of catabolite repression. The following observations were made. Firstly, mutant cells were found to be partially resistant to the inhibitory effect of glucosamine. Secondly, slow growth induced by glucosamine resulted in cells becoming larger, in direct contrast to the effect of slowing growth by glucose limitation in a chemostat or by carbon source substitution. It is concluded that the level of repression/derepression, rather than absolute growth rate, is responsible for controlling cell size. Thirdly, when WHI2 transcript levels were measured it was found that expression was correlated with growth rate rather than the level of repression. These results are interpreted in terms of a model which envisages that the WHI2 product acts as a negative regulator of catabolite repression. A test of this model is reported: it is shown that mutant cells respired more actively in the presence of glucose and grew more rapidly on glycerol, whereas overexpression of WHI2 from multicopy plasmids prevented growth on glycerol and depressed respiration.