A high degree of homology exists between the protein encoded by SLG and the S receptor domain encoded by SRK in self-incompatible brassica campestris l.

The sporophytic self-incompatibility system in Brassica campestris is controlled by multiple alleles of a single locus, S. We isolated and characterized cDNA clones that correspond to genes for S-glycoprotein (SLG) and S-receptor kinase (SRK) from an S⁹-homozygous strain. An SRK⁹ fragment was amplified by the polymerase chain reaction (PCR) by use of primers that corresponded to regions of the gene for the extracellular receptor domain (S-domain) and the kinase domain. The fragment was used as a probe to isolate an SRK⁹ cDNA clone from the cDNA library of stigmas of an S⁹-homozygote. An SLG⁹ cDNA clone was also isolated from the library by use of SLG⁸ cDNA as a probe. Restriction fragment length polymorphism (RFLP) linkage analysis of the F₂ plants from an S⁸S⁹ hybrid demonstrated that the SRK and SLG genes were tightly linked to one another and were also linked to S-genotypes, as determined by pollination tests. The transcripts of SRK⁹ and SLG⁹ were detected in stigmas, but not in anthers or leaves. The cytoplasmic kinase domain encoded by SRK⁹ contained amino acid sequences that are conserved in serine/threonine kinases. The nucleotide sequence encoding the S-domain of SRK⁹ was 98.4% homologous to that of SLG⁹ at the nucleotide level. This observation suggests the existence of a mechanism for maintaining a high degree of similarity between SLG and the region that encodes the S-domain of SRK.