Comparison of an S-protein expression between self-compatible and -incompatible Japanese pear cultivars

An S-4-allele-associated protein (S-4-protein) was identified by both isoel ectricfocusing (IEF) polyacrylamide-gel electrophoresis and N-terminal amin o-acid sequences in self-incompatible Nijisseiki (self-incompatibility geno type=S2S4) and self-compatible Osa-Nijisseiki (S2S4SM, SM=stylar-part mutan t) styles of the Japanese pear, Pyrus serotina Rehd. var. culta Rehd. Expre ssion of the protein in the cultivars was compared during flower bud develo pment and at post-transcriptional levels. In mature styles, S-4-protein could be detected on IEF gel in both cultivar s and the N-terminal amino acid sequences were identical, although Osa-Niji sseiki contained only one-third the amount contained in Nijisseiki. No diff erence was observed in S-2-protien amounts. In Nijisseiki styles, S-4-prote in was already detectable 8 days before anthesis (DBA) and it was synthesiz ed consistently until 2 days after anthesis (DAA); the amount increased 4.7 -fold during these 10 days. In contrast, S-4-protein in Osa-Nijisseiki was not detected earlier than 6 DBA; a small amount was found at 4 DBA, and it increased gradually as flowers developed. Thus, expression of Osa-Nijisseik i S-4-protein is developmentally controlled in the same way as that of Niji sseiki S-4-protein, but with a time lag of several days; the protein level at 2 DAA corresponded to that of Nijisseiki earlier than 4 DBA. S-4-protein s from both Nijisseiki and Osa-Nijisseiki showed RNase activity and the act ivity was also developmentally controlled; it increased about fourfold duri ng the interval from 8 DBA to 2 DAA in Nijisseiki, and 3.3-fold during the interval from 4 DBA to 2 DAA in Osa-Nijisseiki. Activity at 2 DAA, however, was twice as high in Nijisseiki. In vitro protein synthesis showed that po ly(A)(+) from mature Osa-Nijisseiki styles could form S-4-like protein in a manner similar to that of Nijisseiki. These results suggest that the self- compatibility of Osa-Nijisseiki is due to a low level of S-4-protein expres sion, a mechanism very similar to the low level of S-protein and weak incom patibility in immature styles of self-incompatible Nijisseiki. Part of this protein repression may be regulated during post-transcriptional events.