Identification of regulatory mutants of Aspergillus aculeatus affected in rhamnogalacturonan hydrolase expression.

Rhamnogalacturonan hydrolase expression in A. aculeatus can be induced by pectin, but also by a combination of two constituent monosaccharides of pectin, rhamnose and galacturonic acid. The rhgA promoter was fused to the A. niger glucose oxidase coding sequence and a single copy of the hybrid gene was integrated at the rhgA locus in the genome of A. aculeatus. The gene product was subsequently used as reporter in a screening assay for the selection of rhamnogalacturonan hydrolase-overproducing mutant strains. At least four of the mutations were recessive and could be assigned to different loci. One mutation (rgr25) showed linkage with the rhgA locus. Inducible rhamnogalacturonan hydrolase expression levels of about 5-10 times that in the wild-type were found in the mutants rgr48, rgr25 and rgr34 after growth on a combination of rhamnose and galacturonic acid with or without fructose as a carbon source. In mutant rgr48 elevated levels of rhgA transcription were found.