Characterization of the flanking regions of the S-RNase genes of Japanese pear (Pyrus serotina) and apple (Malus x domestica)

Genomic sequences of the self-incompatibility genes, the S-RNase genes, from two rosaceous species, Japanese pear and apple, were characterized. Genomic Southern blot and sequencing of a 4.5-kb genomic clone showed that the S⁴-RNase gene of Japanese pear is surrounded by repetitive sequences as in the case of the S-RNase genes of solanaceous species. The flanking regions of the S²- and S(f)-RNase genes of apple were also cloned and sequenced. The 5' flanking regions of the three alleles bore no similarity with those of the solanaceous S-RNase genes, although the position and sequence of the putative TATA box were conserved. The putative promoter regions of the Japanese pear S⁴- and apple S(f)-RNase genes shared a stretch of about 200 bp with 80% sequence identity. However, this sequence was not present in the S²-RNase gene of apple, and thus it may reflect a close relationship between the S⁴- and S(f)-RNase genes rather than a cis-element important in regulating gene expression. Despite the uniform pattern of expression of the rosaceous S-RNase genes, sequence motifs conserved in the 5' flanking regions of the three alleles were not found, implying that the cis-element controlling pistil specific gene expression also locates at the intragenic region or upstream of the analyzed promoter region.