Summary: We have devised a convenient procedure to induce the yeast-to-mycelium transition of in conditions which avoid the occurrence of the reverse process during the period of study. Yeast cells in late exponential phase were resuspended in water and cooled down to 4 °C for at least 15 min, then heat-shocked by inoculation into a pre-warmed (30 °C) medium containing -acetyl-D-glucosamine. Under these conditions, yeast cells developed into large branching filaments which continued elongating for more than 24 h. Further, ornithine decarboxylase (ODC) activity and polyamine cell pools increased compared to those of cells maintained in glucose medium, which continued yeast-like growth. Addition of ODC inhibitors blocked mycelial development, but only if added during a critical initial period after which they had no effect. At effective concentrations, ODC inhibitors had no significant effect on cell growth. Comparative studies of intact and permeabilized cells suggest that this selective effect is probably due to the location of ODC in more than one cell compartment, one of them being inaccessible to the drugs. Blocking of the morphological transition by ODC inhibitors was specifically reversed by putrescine, and by growing the cells in the presence of 5-azacytidine. It is suggested that the effect of the latter compound is related to its capacity to inhibit DNA methylation, indicating a relationship between polyamines and DNA methylation at the onset of the differentiation process.


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