STCS, A PUTATIVE P-450 MONOOXYGENASE, IS REQUIRED FOR THE CONVERSION OF VERSICOLORIN-A TO STERIGMATOCYSTIN IN ASPERGILLUS-NIDULANS

Sterigmatocystin (ST) and aflatoxin are carcinogenic end point metabol ites derived from the same biochemical pathway, which is found in seve ral Aspergillus spp. Recently, an ST gene cluster, containing approxim ately 25 distinct genes that are each proposed to function specificall y in ST biosynthesis, has been identified in Aspergillus nidulans. Eac h of these structural genes is named stc (sterigmatocystin) followed b y a consecutive letter of the alphabet. We have previously described s tcU (formerly verA) as encoding a keto-reductase required for the conv ersion of versicolorin A to ST. We now describe a second A. nidulans g ene, stcS (formerly verB), that is located within 2 kb of stcU in the ST gene cluster. An stcS-disrupted strain of A. nidulans, TSS17, was u nable to produce ST and converted ST/aflatoxin precursors to versicolo rin A rather than ST, indicating that stcS functions at the same point in the pathway as stcU, Genomic sequence analysis of stcS shows that it encodes a cytochrome P-450 monooxygenase and constitutes a novel P- 450 family, CYP59, Assuming that StcU activity mimics that of similar P-450s, it is likely that StcU catalyzes one of the proposed oxidation steps necessary to convert versicolorin A to ST, These results consti tute the first genetic proof that the conversion of versicolorin A to ST requires more than one enzymatic activity.