A ribonuclease, with a molecular mass of 30 kDa and a potent inhibitory activity toward HIV-1 reverse transcriptase (IC50 = 300 nM), was isolated from dried fruiting bodies of the edible wild mushroom Thelephora ganbajun. The ribonuclease exhibited a unique polyhomoribonucleotide specificity, with the highest activity toward poly(U), about 50% and 25% as much activity toward poly(A) and poly(C), respectively, and minimal activity toward poly(G). Unlike other mushroom RNases, the ribonuclease was adsorbed on DEAE-cellulose and Q-Sepharose, and unadsorbed on CM-cellulose. A temperature of 40 degreesC and a pH of 6-7 were required for maximal activity of the enzyme. The enzyme was characterized by an N-terminal sequence without any homology to known proteins. (C) 2004 Elsevier Inc. All rights reserved.