The S-locus of Nicotiana alata: genomic organization and sequence analysis of two S-RNase alleles

Genomic clones encoding the S2- and S6-RNases of Nicotiana alata Link and Otto, which are the allelic stylar products of the self-incompatibility (S) locus, were isolated and sequenced. Analysis of genomic DNA by pulsed-field gel electrophoresis and Southern blotting indicates the presence of only a single S-RNase gene in the N. alata genome. The sequences of the open-reading frames in the genomic and corresponding cDNA clones were identical. The organization of the genes was similar to that of other S-RNase genes from solanaceous plants. No sequence similarity was found between the DNA flanking the S2- and S6-RNase genes, despite extensive similarities between the coding regions. The DNA flanking the S6-RNase gene contained sequences that were moderately abundant in the genome. These repeat sequences are also present in other members of the Nicotianae.