MOLECULAR-CLONING AND NUCLEOTIDE-SEQUENCES OF CDNAS ENCODING S-ALLELESPECIFIC STYLAR RNASES IN A SELF-INCOMPATIBLE CULTIVAR AND ITS SELF-COMPATIBLE MUTANT OF JAPANESE PEAR, PYRUS PYRIFOLIA-NAKAI

The genes encoding three RNases were cloned from the style of a self- incompatible cultivar, Nijisseiki (S2S4), and its self-compatible muta nt, Osa-Nijisseiki (S2S4sm, sm means stylar part mutant), of Japanese pear. For Nijisseiki, cDNAs coding for two S-RNases (S-2-RNase and S-4 -RNase) and an RNase unrelated to self-incompatibility (non-S-RNase) w ere cloned from the stylar cDNA library. The cDNAs coding for S-2-RNas e, S-4-RNase, and non-S-RNase include 678-, 684-, and 681-bp open read ing frames, respectively. Their deduced amino acid sequences were comp osed of signal peptides and mature RNases (201-203 residues) which wer e verified by partial amino acid sequencing. The primary structures of mature proteins revealed that these RNases are of the RNase T-2 type; only the two S-RNases have several potential N-glycosylation sites an d 60% of their amino acid residues are identical, compared with 25% se quence identity with the non-S-RNase. Such a distinct difference in th e primary structures between S-RNases and non-S-RNase has not previous ly been reported and may be a feature typical of S-RNases in the famil y Rosaceae. Similar experiments were performed for Osa-Nijisseiki. The cDNAs coding for S-2-RNase and non-S-RNase were similarly cloned from the stylar cDNA library. However, the cDNA coding for S-4-RNase was n either amplified by PCR nor cloned from the library, suggesting that t he mutation of self- incompatible Nijisseiki to self-compatible Osa-Ni jisseiki is due to a failure of expression of S-4-RNase. These results lead to the idea that Osa-Nijisseiki is a variant of Nijisseiki in wh ich the S-4-allelic gene in the S-locus is exclusively mutated or dele ted, causing severely impaired or suppressed expression of its gene pr oduct, S-4-RNase, at the style.