Amino-acid sequence of glycoproteins encoded by three alleles of the S locus of Brassica oleracea

Abstract

The pollen–stigma interaction of self-incompatibility in the mustard family Brassicaceae is controlled by the S locus. Nearly fifty allelic variants at this locus control the specificity of this interaction. Identity of alleles in pollen and stigma results in an incompatible reaction and the arrest of pollen development. Genetic and developmental analyses of self-incompatibility in Brassica have identified a class of stigma glycoproteins exhibiting allele-specific isoforms as being among the products of the S-locus^1–3. A cDNA clone encoding one of these S-locus specific glycoproteins has been isolated from B. oleracea⁴, and recently, N-terminal protein sequence analysis has shown extensive similarity between three S-locus specific glycoproteins from B. campestris⁵. Here, we describe cDNA clones encoding such glycoproteins from three different S-allele homozygous genotypes, and report the complete amino-acid sequence of the glycoproteins based on the nucleotide sequence of the cDNA clones. We identify allele-encoded differences in the protein sequences which may be relevant to the allelic specificity of the stigma component of the pollen–stigma interaction. The sequences from the sporophytically determined incompatibility system of Brassica show no similarity to the reported S-locus-associated cDNA sequence of the gametophytic Nicotiana self-incompatibility type⁶.