Identification of a non-S RNase, a possible ancestral form of S-RNases, in Prunus

This study identifies and characterizes a basic non-S RNase in the styles with stigmas of sweet cherry (Prunus avium L.), a member of the Rosaceae subfamily Amygdaloideae, which has an RNase-based gametophytic self-incompatibility system. Internal sequences of putative non-S RNases (RNase PA1 and PA2) were determined, and a cDNA for PA1 was obtained. The deduced amino acid sequence of PA1 contained two conserved sequence motifs essential for T2/ S-type RNase activity. PA1 shows 20-30% sequence identity to S-RNases of Rosaceae, Solanaceae and Scrophulariaceae, and non-S RNases of higher plants. Transcription of the PA1 gene was specific to the styles with stigmas, and the gene was not expressed in other tissues. Although PA1 resembles RNase X2, a non-S RNase from Petunia inflata, the placement of PA1 and RNase X2 in the phylogenetic tree was quite different. Placement of PA1 was also distinct from that of rosaceous S-RNases, while RNase X2 was incorporated in the clade of S-RNases from the Solanaceae. The sole intron in the PA1 gene is located at a position equivalent to that of the second intron of amygdaloid S-RNase genes, and that of the only intron in most other S-RNase genes. Genomic Southern analysis revealed the presence of sequences homologous to PA1 in all of the other four Prunus species tested, suggesting that PA1 has an important physiological function. The significance of the discovery of PA1 is discussed in terms of the origin and evolution of S-RNases and self-incompatibility in Rosaceae.