Candida albicans, the most common fungal pathogen, regulates its cellular m
orphology in response to environmental conditions. The ODC gene, which enco
des ornithine decarboxylase, a key enzyme in polyamine biosynthesis, was is
olated and disrupted. Homozygous null Candida mutants behaved as polyamine
auxotrophs and grew exclusively in the yeast form at low polyamine levels (
0.01 mM putrescine) under all conditions tested. An increase in the polyami
ne concentration (10 mM putrescine) restored the capacity to switch from th
e yeast to the filamentous form. The strain with a deletion mutation also s
howed increased sensitivity to salts and calcofluor white, This Candida odc
/odc mutant was virulent in a mouse model. The results suggest a model in w
hich polyamine levels exert a pleiotrophic effect on transcriptional activi
ty.