The physiological function of a microsomal cytochrome P450, P450rm, from Rhodotorula minuta was determined. We obtained a mutant, 5-286, which has a point mutation in the P450rm gene, resulting in loss of the activities of the isobutene-forming enzyme and benzoate 4-hydroxylase and the ability to grow on medium containing L-phenylalanine as the sole carbon source. Mutant 5-286 spontaneously reverted to the wild-type at a frequency of approximately 10(-9)-10(-10). These results indicate that P450rm functions in the dissimilation pathway starting from L-phenylalanine.