cDNA Cloning, Sequence Analysis and Homology Modeling of S20-RNase in Chinese White Pear (Pyrus bretschneideri Rehd.)

To elucidate the function of S-allele and the possible molecular mechanism of gametophytic self-incompatibility (GSI) in Chinese white pear (Pyrus bretschneideri Rehd.), full length cDNA encoding S₂₀-RNase was cloned by rapid amplification of cDNA ends (RACE) approach from the cultivar ´Donghuang´ (S₈S₂₀), and analyzed by bioinformatics method. S₂₀-RNase displayed typical sequence features of pear S₂₀-RNases, i.e. five conserved regions and a hypervariable (HV) region. The predicted three-dimension (3D) structure of the S₂₀-RNase was similar to that of 53-RNase of Japanese pear (P. pyrifolia), indicating that the S₂₀-RNase gene behaves as RNase rejecting pollen tube growth with the same S-haplotype as S₃-RNase does in GSI response. At the amino acid level, S₂₀-RNase showed 31.5% to 90.8% similarities with other rosaceous S-RNases. Phylogenetic analysis revealed that rosaceous S-RNases occurred before divergence of species, but after divergence of subfamilies Maloideae and Amygdaloideae.