Expression of the Escherichia coli beta-glucuronidase gene in Pseudocercosporella herpotrichoides.

The plant-pathogenic fungus Pseudocercosporella herpotrichoides has been successfully transformed by using two different positive selection systems in combination with the Escherichia coli gusA gene. The selectable markers used in this study were the hygromycin B phosphotransferase gene (hph) from E. coli and the gene (bml) for beta-tubulin from a benomyl-resistant mutant of Neurospora crassa. A lower transformation rate was obtained with the bml system than with the hph system. Conversely, cotransformation frequencies, as determined with medium plates containing the chromogenic substrate 5-bromo-4-chloro-3-indolyl-beta-D-glucuronic acid, were higher with bml than with hph as the selectable marker. The hygromycin-resistant transformants were mitotically stable, and both the selectable gene and gusA were maintained through conidiation. The vector DNA was integrated into the genome, and the number and sites of insertion varied among transformants. Enzyme assays of mycelial extracts showed that beta-glucuronidase activity was highest in transformants with a high gusA copy number. Expression of gusA during growth of the fungus on plants was easily detectable and did not affect pathogenicity. These results form the basis for construction of a versatile and sensitive reporter gene system for P. herpotrichoides.