Mutualistic fungal endophytes express a proteinase that is homologous to proteases suspected to be important in fungal pathogenicity

Many cultivated and wild grass species are hosts to mutualistic fungal endophytes. These associations are ecologically and agronomically significant, yet little is known regarding the physiological aspects of the interaction. In the Poa ampla/Acremoniom typhinum interaction, a fungal serine proteinase, Atl, is surprisingly abundant and may constitute 1 to 2% of the total leaf-sheath protein. Sequence analysis of cDNA and genomic clones indicates that proteinase Atl is a member of the eukaryotic subtilisin-like protease family. It is homologous to proteases suspected to be virulence factors in fungal pathogens of insects, nematodes, and other fungi. Cel blot analysis of RNA extracted from infected leaf-sheath tissue indicates that the proteinase Atl transcript level is extremely high. RNA gel blots and immunoblots of purified enzymes indicate that similar proteinases are produced by Epichloe festucae and Acremonium lolii, the fungal endophytes infecting Festuca rubra subsp. rubra and Lolium perenne, respectively. Fungal expression of proteinase Atl-like enzymes may be a general feature of endophyte infection.