A novel beta-glucosidase in Uromyces fabae: feast or fight?

Efficient nutrient mobilization is a key element for biotrophic plant parasites such as the rust fungi. In the course of a cDNA library screen for elements involved in sugar utilization in Uromyces fabae, we identified a sequence with homology to beta-glucosidases. Full-length genomic and cDNA clones of the gene, termed BGL1, were isolated and sequenced. The BGL1 gene comprises 3,372 nucleotides, including nine introns. The open reading frame encompasses 2,532 bases and codes for a polypeptide of 843 amino acids with an apparent molecular mass of 92.4 kDa. Analysis of the polypeptide revealed a potential secretion signal, indicating an extracellular localization of mature BGL1p (89.8 kDa). BGL1 seems to be expressed in all stages of growth, including haustoria, the feeding structures of rust fungi. In the course of immunolocalization studies, the gene product BGL1p was localized in the periphery of intercellular hyphae and haustoria. On the basis of sequence homology, the BGL1 gene was identified as a fungal beta-glucosidase.