Genomic organization of the S locus: Identification and characterization of genes in SLG/SRK region of S⁹ haplotype of Brassica campestris (syn. rapa)

In Brassica, two self-incompatibility genes, encoding SLG (S locus glycoprotein) and SRK (S-receptor kinase), are located at the S locus and expressed in the stigma. Recent molecular analysis has revealed that the S locus is highly polymorphic and contains several genes, i.e., SLG, SRK, the as-yet-unidentified pollen S gene(s), and other linked genes. In the present study, we searched for expressed sequences in a 76-kb SLG/SRK region of the S⁹ haplotype of Brassica campestris (syn. rapa) and identified 10 genes in addition to the four previously identified (SLG⁹, SRK⁹, SAE1, and SLL2) in this haplotype. This gene density (1 gene/5.4 kb) suggests that the S locus is embedded in a gene-rich region of the genome. The average G + C content in this region is 32.6%. An En/Spm-type transposon-like element was found downstream of SLG⁹. Among the genes we identified that had not previously been found to be linked to the S locus were genes encoding a small cysteine- rich protein, a J-domain protein, and an antisilencing protein (ASF1) homologue. The small cysteine-rich protein was similar to a pollen coat protein, named PCP-A1, which had previously been shown to bind SLG.