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Deletion of the Gibberella fujikuroi glutamine synthetase gene has significant impact on transcriptional control of primary and secondary metabolism

In Gibberella fujikuroi, the gibberellin (GA) and bikaverin biosynthesis are under control of nitrogen metabolite repression. However, the signalling components acting upstream of AREA are still unknown. We investigated the role of glutamine synthetase (GS) both as an enzyme and as a possible regulator in the nitrogen regulation system. We cloned and replaced the GS‐encoding gene, glnA‐Gf. The mutants grow with a phenotype different from the wild type in the presence of glutamine. They were unable to express nitrogen‐repressed GA and bikaverin biosynthetic genes even under nitrogen starvation conditions. Complementation with the glnA‐Gf wild‐type copy fully restored GS activity, the expression of secondary metabolism genes, and the production of GAs and the red pigment, bikaverin. In order to find more target genes of GS, differential cDNA‐screening and differential hybridization of macroarrays were performed using cDNA from the wild type and ΔglnA mutant as probes. Several genes were dramatically up‐ or downregulated in the mutant. Among them are genes involved in N‐ and C‐catabolism, and in transcriptional and translation control. Some of these genes are also under AREA control. Treatment with the GS inhibitor l‐methionine sulphoximine resulted in similar expression patterns as in the glnA mutant with ammonium as nitrogen source, whereas glutamine can overcome the up‐ or downregulation of most but not all of the target genes. These findings suggest that not only glutamine, but also GS itself might play an important role in nitrogen metabolite repression.