Protein O-mannosyltransferases are required for sterigmatocystin production and developmental processes in Aspergillus nidulans

Aspergillus nidulans produces sterigmatocystin (ST), a precursor of a carcinogenic secondary metabolite aflatoxin (AF), during its developmental process. ST biosynthesis has been shown to be affected by various regulatory factors. In this study, we investigated the involvement of O-mannosyltransferases (PmtA, PmtB, PmtC), in ST production and morphological development. Deletion of pmtA (pmtA), pmtB (pmtB) or pmtC (pmtC) caused no spore production and a significant decline of vegetative growth. A tremendous decline of ST level was observed in all pmt mutants at the third day after inoculation. By extending the growth period, ST production of pmtA and pmtB increased to the wild-type level 7days after inoculation. On the other hand, ST was not detected from 7- or 14-day cultures in pmtC. Expression levels of aflR gene, an essential regulator of the ST biosynthesis pathway, were also down-regulated in the pmt strains. By introducing the aflR overexpression cassette, ST production in the pmtA and pmtB significantly increased to levels comparable to the wild type. However, the presence of the aflR overexpression cassette could not improve ST production in the pmtC mutant. These data suggest that the PMT family is a new endogenous factor that is required for ST biosynthesis in A. nidulans. These findings provide better understanding of the regulatory mechanisms of AF/ST biosynthesis, which can ultimately contribute to our ability to control aflatoxin contamination.