The preparation and analysis of a mutant ribonuclease (RNase) T1 which possesses higher nucleolytic activity than the wild-type enzyme are described. The gene for the mutant RNase T1 (Tyr45 -+ Trp45), in which a single amino acid at the binding site of the guanine base has been changed, was constructed by the cassette mutangenesis method using a chemically synthesized gene Proc. Natl Acad. Sci. U S A 83,4695 -46991. In order to reduce the nucleolytic activity of the enzyme in vivo, this gene was expressed in Escherichia coli as a fused protein connected through methionine residues to other proteins at both the N-and C-termini. After liberation from the fused protein by cleavage with cyanogen bromide at the methionine junctions, the mutant RNase T1 was purified by column chromatography. The nucleolytic activity toward pGpC increased to 120% of that of wild-type RNase T I . The kinetic parameters of the mutant enzyme demonstrate that this higher nucleolytic activity is due to a higher affinity for the substrate, probably because of an increased stacking effect in the binding pocket for the guanine base. This mutant enzyme also possessed a higher nucleolytic activity against pApC than wild-type RNase T1.Ribonuclease T1 (RNase T,) from Aspergillus oryzae specifically hydrolyzes the phosphodiester linkages of guanosine 3'-phosphate residues in single-stranded RNA [l]. This enzyme is indispensable for the sequencing of RNA as is RNase A which is specific for pyrimidine bases. RNase T1 has also been studied extensively because of its high substrate specificity, its stability and relatively easy purification [2, 31.The nature of the binding site for the guanine base was elucidated from the X-ray crystallographic analysis of an RNase-TI -2'GMP complex [4, 51. It appears that the guanine base is bound to the enzyme by four hydrogen bonds with amino acid residues Asn43, Asn44, Glu46 and Asn98, and also by stacking with Tyr42 and Tyr45 (Fig. 1). Of these two tyrosine residues, Tyr42 is located inside the enzyme molecule and forms, with Phe100,lle90 and Phe48, the bottom of a binding pocket for the guanine base (Fig. lc). Tyr45 is located on the surface of the enzyme and acts as a 'lid' for the binding pocket (Fig. 1 b). A couple of model systems of molecular recognition have been studied recently by analogy with substrate recognition involving hydrogen bonds and stacking in RNase TI [6, 71. Recently we constructed the gene for RNase T1 from chemically synthesized deoxyribooligonucleotides and sucCorrespondence to S. Nishikawa,