CDNA CLONING AND MOLECULAR ANALYSIS OF 2 SELF-INCOMPATIBILITY ALLELESFROM APPLE

Complementary DNA clones representing two alleles of the self-incompat ibility (S) locus of apple (Malus x domestica Borkh.) have been isolat ed and characterised. One of the alleles corresponds to a 29 kDa ribon uclease (S-RNase) that was purified from pistil tissue. On northern bl ots, both cDNAs hybridized to a transcript that was only present in pi stils and not in the other plant tissues analysed. Corresponding genom ic sequences, amplified by PCR, were found to contain a single intron of 138 bp and 1100 bp respectively. Comparison of both sequences shows that the cDNAs encode mature proteins containing 65% of identical res idues. Eight invariable cysteine residues, conserved regions around tw o histidines thought to play a role in RNA catalysis, and a number of other distinct residues are conserved between the apple S-RNases and s imilar proteins in the family Solanaceae. As this is the first report of sequences of S-alleles from a species belonging to a family that is not related with the Solanaceae, the structural features of S-RNases deduced from a comparison of their sequences are discussed.