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 S-9-homozygous strain. An SRK(9) 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 SRK(9) cDNA clone from the cDNA library of stigmas of an S-9-homozygote. An SLG(9) cDNA clone was also isolated from the library by use of SLG(8) cDNA as a probe. Restriction fragment length polymorphism (RFLP) linkage analysis of the F-2 plants from an (SS9)-S-8 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 SRK(9) and SLG(9) were detected in stigmas, but not in anthers or leaves. The cytoplasmic kinase domain encoded by SRK(9) contained amino acid sequences that are conserved in serine/threonine kinases. The nucleotide sequence encoding the S-domain of SRK(9) was 98.4% homologous to that of SLG(9) 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.