Although the S-like ribonucleases (RNases) share sequence homology with the S-RNases involved in the self-incompatibility mechanism in plants, they are not associated with this mechanism. They usually function in stress responses in non-carnivorous plants and in carnivory in carnivorous plants. In this study, we clarified the structures of the S-like RNases of Aldrovanda vesiculosa, Nepenthes bicalcarata and Sarracenia leucophylla, and compared them with those of other plants. At ten positions, amino acid residues are conserved or almost conserved only for carnivorous plants (six in total). In contrast, two positions are specific to non-carnivorous plants. A phylogenetic analysis revealed that the S-like RNases of the carnivorous plants form a group beyond the phylogenetic relationships of the plants. We also prepared and characterized recombinant S-like RNases of Dionaea muscipula, Cephalotus follicularis, A. vesiculosa, N. bicalcarata and S. leucophylla, and RNS1 of Arabidopsis thaliana. The recombinant carnivorous plant enzymes showed optimum activities at about pH 4.0. Generally, poly(C) was digested less efficiently than poly(A), poly(I) and poly(U). The kinetic parameters of the recombinant D. muscipula enzyme (DM-I) and A. thaliana enzyme RNS1 were similar. The k cat/Km of recombinant RNS1 was the highest among the enzymes, followed closely by that of recombinant DM-I. On the other hand, the kcat/Km of the recombinant S. leucophylla enzyme was the lowest, and was ~1/30 of that for recombinant RNS1. The magnitudes of the kcat/Km values or kcat values for carnivorous plant S-like RNases seem to correlate negatively with the dependency on symbionts for prey digestion.
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