Functional analysis of the carS gene of Fusarium fujikuroi.

The ascomycete fungus Fusarium fujikuroi is a model system in the investigation of the biosynthesis of some secondary metabolites, such as gibberellins, bikaverin, and carotenoids. Carotenoid-overproducing mutants, generically called carS, are easily obtained in this fungus by standard mutagenesis procedures. Here we report the functional characterization of gene carS, responsible for this mutant phenotype. The identity of the gene was demonstrated through the finding of mutations in six independent carS mutants and by the complementation of one of them. The F. fujikuroi carS gene was able to restore the control of carotenogenesis in a similar deregulated mutant of Fusarium oxysporum, but only partially at the transcription level, indicating an unexpected complexity in the regulation of the pathway. Due to the pleiotropic characteristics of this mutation, which also modifies the production of other secondary metabolites, we did a screening for carS-regulated genes by subtracted cDNA hybridization. The results show that the carS mutation affects the regulation of numerous genes in addition to those of carotenogenesis. The expression of the identified genes was usually enhanced by light, a regulatory effect also exhibited by the carS gene. However, in most cases, their mRNA levels in carS mutants were similar to those of the wild type, suggesting a regulation that affects mRNA availability rather than mRNA synthesis.