Proline-induced germ-tube formation in Candida albicans: role of proline uptake and nitrogen metabolism.

Proline-induced germ-tube formation and cell-cell aggregation in four strains of Candida albicans were completely inhibited when the pH of the medium was 5.0 or lower, whereas morphogenesis induced by N-acetylglucosamine (GlcNAc) was unaffected even at pH 4.5. The pH sensitivity of proline-induced germ-tube formation was not caused by a modulation of proline uptake, which was unchanged over the pH range 4.5-6.5. The proline uptake system was specific, constitutive and subject to ammonium repression, and only one permease was detected, with a Km of 179 microM. Cultures deprived of nitrogen in the presence of glucose were derepressed for proline uptake but the yeast-mycelial transition could not be mediated by either proline or GlcNAc. The inhibition of morphogenesis was reversed when the nitrogen starvation was relieved by the addition of ammonium ions, proline, or certain amino acids. These results indicate that the nitrogen status of the cells is critical for the morphogenesis of C. albicans.