Control of morphogenesis, protease secretion and gene expression in Candida albicans by the preferred nitrogen source ammonium
Micro-organisms sense the availability of nutrients in their environment to control cellular behaviour and the expression of transporters and enzymes that are required for the utilization of these nutrients. In the pathogenic yeast Candida albicans, the preferred nitrogen source ammonium suppresses the switch from yeast to filamentous growth in response to certain stimuli, and it also represses the secretion of proteases, which are required for the utilization of proteins as an alternative nitrogen source. To investigate whether C. albicans senses the availability of ammonium in the extracellular environment or if ammonium uptake into the cell is required to regulate morphogenesis and gene expression, we compared the behaviour of wild-type cells and ammonium uptake-deficient mutants in the presence and absence of extracellular ammonium. Arginine-induced filamentous growth was suppressed by ammonium in the wild-type, but not in mutants lacking the ammonium permeases Mep1 and Mep2. Similarly, ammonium suppressed protease secretion and extracellular protein degradation in the wild-type, but not in mutants lacking the ammonium transporters. By comparing the gene expression profiles of C. albicans grown in the presence of low or high ammonium concentrations, we identified a set of genes whose expression is controlled by nitrogen availability. The repression of genes involved in the utilization of alternative nitrogen sources, which occurred under ammonium-replete conditions in the wild-type, was abrogated in mep1 mep2 mutants. These results demonstrate that C. albicans does not respond to the presence of sufficient amounts of the preferred nitrogen source ammonium by sensing its availability in the environment. Instead, ammonium has to be taken up into the cell to control morphogenesis, protease secretion and gene expression.