Role of the Fusarium fujikuroi TOR Kinase in Nitrogen Regulation and Secondary Metabolism

Abstract

In Fusarium fujikuroi, the biosynthesis of gibberellins (GAs) and bikaverin is under control of AreA-mediated nitrogen metabolite repression. Thus far, the signaling components acting upstream of AreA and regulating its nuclear translocation are unknown. In Saccharomyces cerevisiae, the target of rapamycin (TOR) proteins, Tor1p and Tor2p, are key players of nutrient-mediated signal transduction to control cell growth. In filamentous fungi, probably only one TOR kinase-encoding gene exists. However, nothing is known about its function. Therefore, we investigated the role of TOR in the GA-producing fungus F. fujikuroi in order to determine whether TOR plays a role in nitrogen regulation, especially in the regulation of GA and bikaverin biosynthesis. We cloned and characterized the F. fujikuroi tor gene. However, we were not able to create knockout mutants, suggesting that TOR is essential for viability. Inhibition of TOR by rapamycin affected the expression of AreA-controlled secondary metabolite genes for GA and bikaverin biosynthesis, as well as genes involved in transcriptional and translational regulation, ribosome biogenesis, and autophagy. Deletion of fpr1 encoding the FKBP12-homologue confirmed that the effects of rapamycin are due to the specific inhibition of TOR. Interestingly, the expression of most of the TOR target genes has been previously shown to be also affected in the glutamine synthetase mutant, although in the opposite way. We demonstrate here for the first time in a filamentous fungus that the TOR kinase is involved in nitrogen regulation of secondary metabolism and that rapamycin affects also the expression of genes involved in translation control, ribosome biogenesis, carbon metabolism, and autophagy.

A Nitrogen Response Pathway Regulates Virulence Functions in Fusarium oxysporum via the Protein Kinase TOR and the bZIP Protein MeaB

“…Here, we showed that rapamycin treatment increases transcript levels of genes involved in nitrogen catabolism, such as Nit1, supporting a role of TOR in nitrogen catabolite repression of F. oxysporum. This result is in agreement with a previous study showing that rapamycin activates expression of nitrogen metabolismrelated genes, such as ammonium permease and uricase in F. fujikuroi (Teichert et al, 2006).…”

“…Similarly, in F. fujikuroi, rapamycin was shown to activate AreA-regulated genes in the wild type but not in the DareA background (Teichert et al, 2006). Moreover, a gln3 gat1 double mutant of S. cerevisiae was blocked in rapamycin-mediated induction of the nitrogen-regulated genes MEP2 and GLN1 and was weakly resistant against rapamycin (Beck and Hall, 1999).…”

“…In contrast with S. cerevisiae, which has two TOR proteins, only a single TOR ortholog was identified in filamentous ascomycetes such as A. nidulans, F. fujikuroi, and Podospora anserina (Dementhon et al, 2003;Fitzgibbon et al, 2005;Teichert et al, 2006 (Poulter et al, 2003). The predicted TOR1b gene product contains three of the four N-terminal HEAT (huntingtin, elongation factor 3, regulatory subunit A of PP2A, TOR1) repeats as well as all the other key domains of TOR proteins, including FAT (FRAP, ATM, TTRAP), FRB (FKBP12-rapamycin binding), catalytic kinase, and C-terminal FATC domains.…”

“…Depletion of the preferred nitrogen source Gln by treatment with the Gln synthetase inhibitor L-methionine sulfoximine (MSX) also caused nuclear localization of Gln3, suggesting that TOR responds to Gln levels . In the filamentous fungus Fusarium fujikuroi, TOR inhibition by rapamycin affected expression of AreA-controlled genes involved in secondary metabolism, translation control, ribosome biogenesis, carbon metabolism, and autophagy (Teichert et al, 2006). The role of TOR in nutrient regulation of fungal virulence on plants has not been examined so far.…”

Control of Aflatoxin Biosynthesis in Aspergilli

“…A number of other protein kinases have been implicated in transduction of the environmental signals including the TOR kinase and MAP kinases. The TOR kinase, characterized in yeast as responsible for regulating nutrient mediated growth signaling, has recently been implicated in the biosynthesis of a fungal secondary metabolite (Teichert et al, 2006). Nutrient regulation of AF and ST biosynthesis may be mediated by cAMP levels.…”