CPR1: a gene encoding a putative signal peptidase that functions in pathogenicity of Colletotrichum graminicola to maize.

Colletotrichum graminicola causes anthracnose leaf blight and stalk rot of maize. We used restriction-enzyme mediated insertional (REMI) mutagenesis to identify a gene in this fungus that is required for pathogenicity to both stalks and leaves. The predicted polypeptide encoded by this gene, which we have named CPR1, is similar to a family of proteins that comprise one subunit of the eukaryotic microsomal signal peptidase. The nonpathogenic CPR1 REMI mutant contains a plasmid integration in the 3' untranslated region of the gene, 19 bp downstream from the stop codon. The result is a significant reduction in transcript levels in comparison to the wild type, perhaps as a result of increased transcript instability. We were unable to knock out the CPR1 gene, and it may be essential for viability. Microscopic examination of the REMI mutant on maize leaves revealed that it is fully capable of penetrating and colonizing host cells during the initial, biotrophic phases of the disease interaction but, unlike the wild type, it appears to be unable to switch to a necrotrophic mode of growth. We suggest that the CPR1 REMI mutant may be unable to secrete sufficient quantities of degradative enzymes to support that transition. The CPR1 REMI mutant provides us with a useful tool for future studies of the role of fungal protein transport in this important stalk rot disease of maize.