We have isolated a genomic clone encoding trichobakin (TBK), a type I ribosome-inactivating protein from the plant Trichosanthes sp. Bac Kan 8-98 (family Cucurbitaceae), by PCR using specific primers designed from the cDNA sequences of alpha-trichosanthin. The sequence encoding mature TBK was constructed in the pET-21d(+) vector for overexpression in Escherichia coli strain BL21(DE3). The recombinant protein was purified to homogeneity by CM-Sepharose chromatography on FPLC with a final yield of about 55 mg/l of culture. The protein has a molecular mass of about 27 kDa, as shown by SDS/PAGE and matrix-assisted laser-desorption ionization MS. It was found that the protein inhibited luciferase mRNA translation in the rabbit reticulocyte cell-free system with an IC(50) value (that which causes a 50% reduction of residual translational activity) of about 3.5 pM. The rRNA N-glycosidase activity of the protein was also proved at the above-mentioned concentration after rRNAs were treated with acid aniline.