Identification of regions in which positive selection may operate in S-RNase of Rosaceae: Implication for S-allele-specific recognition sites in S-RNase

stylar S-RNase is associated with gametophytic self-incompatibility in the Rosaceae, Solanaceae, and Scrophulariaceae, This S-RNase is responsible for S-allele-specific recognition in the self-incompatible reaction, but how i t functions in specific discrimination is not clear. Window analysis of the numbers of synonymous (d(S)) and non-synonymous (d(N)) substitutions in ro saceous S-RNases detected four regions with an excess of d(N) over d(S) in which positive selection may operate (PS regions). The topology of the seco ndary structure of the S-RNases predicted by the PHD method is very similar to that of fungal RNase Rh whose tertiary structure is known. When the seq uences of S-RNases are aligned with the sequence of RNase Rh based on the p redicted secondary structures, the four PS regions correspond to two surfac e sites on the tertiary structure of RNase Rh, These findings suggest that in S-RNases the PS regions also form two sites and are candidates for the r ecognition sites for S-allele-specific discrimination. (C) 1998 Federation of European Biochemical Societies.