A new MFS-transporter gene next to the gibberellin biosynthesis gene cluster of Gibberella fujikuroi is not involved in gibberellin secretion

The genes of the gibberellin (GA) biosynthesis pathway in Gibberella fujikuroi are organized in a gene cluster consisting of at least seven genes. Here we report the cloning and characterization of smt, a gene encoding a membrane transporter of the major facilitator superfamily I, which is located next to the GA gene cluster. Since pathway-specific transporters occur frequently in prokaryotic and fungal antibiotic and toxin clusters, smt was thought to be involved in GA secretion. The gene is expressed in mycelium grown under GA-production conditions, but not when the GA biosynthesis is repressed by high amounts of ammonium. To investigate the function of SMT, gene replacement experiments were performed. The smt-mutants did not show any reduction in the GA yield; and gibberellic acid or its precursors did not influence the gene expression. However, sugar alcohols, such as myo-inositol, sorbitol and mannitol, induced the expression of smt. The results demonstrate that the smt gene does not play an essential role in biosynthesis and secretion of GAs in G. fujikuroi, despite the location adjacent to the GA gene cluster.