Expression and antisense inhibition of transgenes in Phytophthora infestans is modulated by choice of promoter and position effects

Procedures were identified for manipulating the expression of genes in the oomycete fungus, Phytophthora infestans. The activities of five putative promoter sequences, derived from the 5' regions of oomycete genes, were measured in transient assays performed in protoplasts and in stable transformants. The sequences tested were from the ham34 and hsp70 genes of Bremia lactucae, the actin-encoding genes of P. infestans and P. megasperma, and a polyubiquitin-encoding gene of P. infestans. Experiments using the GUS reporter gene (encoding beta-glucuronidase) demonstrated that each 5' fragment had promoter activity, but that their activities varied over a greater than tenfold range. Major variation was revealed in the level of transgene expression in individual transformants containing the same promoter::GUS or promoter::lacZ fusion. The level of expression was not simply related to the number of genes present, suggesting that position effects were also influencing expression. Fusions between the ham34 promoter, and full-length and partial GUS genes in the antisense orientation blocked the expression of GUS in protoplasts and in stable transformants.