A defined level of protein disulfide isomerase expression is required for optimal secretion of thaumatin by Aspergillus awamori

Thaumatin, a 22-kDa protein containing eight disulfide bonds, is secreted b y the filamentous fungus Aspergillus awamori at levels which are dependent upon the extent of overexpression of protein disulfide isomerase (PDIA). Ad ditional copies of the PDIA-encoding gene pdiA were introduced into a strai n of A. awamori that expresses a cassette encoding thaumatin. Transformants with different levels of pdiA mRNA and measured PDIA levels were chosen fo r examination of the impact that PDIA levels had on thaumatin secretion. Th e secretion of two native proteins, alpha -amylase and acid phosphatase, wa s also examined in relation to varying levels of PDIA. Over a range of PDIA levels of 1-8, relative to the native level in strains with just one copy of the pdiA gene, the fraction of alpha -amylase and acid phosphatase in th e total secreted protein was unaffected. In contrast, a peak level of thaum atin, about 5-fold higher than in the strain with one copy of pdiA, was fou nd in strains with a relative PDIA level of between two and four. Improved thaumatin production was confirmed in 5-1 fermenters using a strain of A. a wamori with six pdiA gene copies, containing 3.2-fold higher levels of PDIA than wild-type strains.