The impact of the ammonium permeases MepA, MepB and MepC on nitrogen-regulated secondary metabolism in Fusarium fujikuroi

Institut für Botanik der Westfälischen Wilhelms-Universität Münster, Schloßgarten 3, D-48149 Münster, Germany; Department of Molecular Genetics and Microbiology, Duke University Medical Center, Durham, North Carolina, USA

^* To whom correspondence should be addressed. Email: tudzynsnb{at}uni-muenster.de.

	Abstract

In Fusarium fujikuroi, the production of gibberellins and bikaverin is repressed by nitrogen sources such as glutamine or ammonium. Sensing and uptake of ammonium by specific permeases play key roles in nitrogen metabolism. Here we describe the cloning of three ammonium permease genes, mepA, mepB, and mepC, and their participation in ammonium uptake and signal transduction in F. fujikuroi. The expression of all three genes is strictly regulated by the nitrogen regulator AreA. Severe growth defects of mepB mutants on low ammonium medium and methylamine uptake studies suggest that MepB functions as the main ammonium permease in F. fujikuroi. In mepB mutants, nitrogen-regulated genes such as the gibberellin and bikaverin biosynthetic genes are derepressed in spite of high extracellular ammonium concentrations. mepA mepB and mepC mepB double mutants show a similar phenotype as mepB mutants. All three F. fujikuroi mep genes fully complemented the Saccharomyces cerevisiae mep1 mep2 mep3 triple mutant to restore growth on low ammonium, whereas only MepA and MepC restored pseudohyphal growth in the mep2/mep2 mutant. Overexpression of mepC in the mepB mutants partially suppressed the growth defect but did not prevent derepression of AreA-regulated genes. These studies provide evidence that MepB functions as a regulatory element in a nitrogen sensing system in F. fujikuroi, yet does not provide the sensor activity of Mep2 in yeast, indicating differences in the mechanisms by which nitrogen is sensed in S. cerevisiae and F. fujikuroi.