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 DeltaglnA 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.