A carotenoid biosynthesis gene cluster in Fusarium fujikuroi:: the genes carB and carRA

Phytoene synthase, phytoene dehydrogenase and carotene cyclase are three of the four enzyme activities needed to produce the acidic carotenoid neurosporaxanthin from the precursor geranylgeranyl pyrophosphate. In the filamentous fungus Fusarium fujikuroi, these three enzyme activities are encoded by two closely linked genes, car RA and car B, oriented in the same direction in the genome. The two genes are separated by 548 bp and code for two polypeptides of 612 and 541 amino acids, respectively, which are highly similar to the homologous proteins from other filamentous fungi. The ORF of car RA contains a 96-bp insertion that is absent in the other fungal homologues. The 32 additional residues are located in one of the two repeated domains responsible for the cyclase activity in the homologous fungal proteins. We have determined the function of car RA by gene disruption. The resulting mutants were albino and had lost the ability to produce phytoene, as expected from the simultaneous loss of phytoene synthase and carotene cyclase. In the same experiments, we also found transformants in which car B had been deleted. These mutants accumulate phytoene, confirming the function of the gene previously shown by gene-targeted mutagenesis. Expression of car RA and car B is strongly induced by light. Loss of car B or disruption of the car RA ORF led to enhanced expression of the car RA gene, suggesting the existence of a feedback regulatory mechanism.