Purification of an antifungal PR-5 protein from flower buds of Brassica campestris and cloning of its gene

Abstract

An antifungal pathogenesis-related (PR) group 5 protein with a molecular mass of 27 kDa (BFTP) was purified from flower buds of Brassica campestris. BFTP exhibits antifungal activity against Neurospora crassa, causing rapid release of cytoplasmic material at the hyphal tips of the fungus. BFTP immune-cross-reacts with antiserum raised against the tobacco osmotin-like PR-5 protein. Using a PCR product generated with the help of two degenerate PCR primers for (1) the N-terminal amino acid sequence of the protein and (2) the conserved peptide domain that appears in all PR-5 proteins, we were able to isolate from a flower-bud cDNA library a cDNA (933 bp) that encodes this protein (pBFTP). The deduced amino acid sequence shows high similarity to PWIR2 from wheat (43%) and thaumatin II from Thaumatococcus daniellii Benth (40%). It contains the 16 cysteine residues that are conserved in all PR-5 proteins at their invariant positions. The cDNA predicts the synthesis of a preprotein which is subsequently processed into the mature protein by removal of an N-terminal signal peptide. The mRNA is predominantly expressed in flower buds, with a moderate level of transcripts detected in stems. Very low levels of the mRNA are present in root and leaf tissue. The gene is a member of a small multigene family in the genome of B. campestris.