The interaction of ribonuclease U-2 (RNAase U-2) with its substrate analogues has been investigated by a gel filtration method. At pH 4.5 and 30 degrees C, the apparent binding strength of the substrate analogues was in the following order; adenylate greater than guanylate greater than inosylate greater than cytidylate among 2'-nucleotides and 2'- greater than 3'- greater than 5'- among adenylate isomers. The formation of an equimolar complex of RNAase U-2 and 2'-nucleotide was indicated from the Scatchard plot. 2. The interaction of RNAase U-2 with 2'-adenylate or 2'-guanylate was observed spectrophotometrically. The complex of RNAase U-2 and 2'-adenylate yielded not only an absorption difference spectrum having a broad positive peak at 280 to 285 nm and a negative trough at 256 nm but also a circular dichroic difference spectrum having a positive peak at around 250 nm and a negative trough at around 290 nm. The complex of RNAase U-2 and 2'-guanylate gave a similar difference spectrum to that of the RNAase T-1 - 3'-guanylate complex, in absorption as well as in circular dichroism.
