CLONING AND EXPRESSION OF A DISTINCTIVE CLASS OF SELF-INCOMPATIBILITY (S) GENE FROM PAPAVER-RHOEAS L

We present the identification, cloning, and characterization of a self-incompatibility (S) gene from Papaver rhoeas that has no significant homology to any previously reported gene sequences, including S genes from other species. This result suggests that a different self-incompatibility mechanism may be operating in this species and has important implications for the evolutionary relationships between the S genes. The S-1 cDNA was cloned by using an oligonucleotide based upon N-terminal amino acid sequence data from stigmatic proteins that show complete linkage with the S-1 gene. The single-copy gene has been expressed in Escherichia coli to test biological activity. Although the recombinant S-1 protein (Sie) is not processed in the same way as the protein produced in the plant, it exhibits, in vitro, the specific pollen inhibitory activity expected of an S gene product; pollen carrying the S-1 allele is inhibited, whereas pollen not carrying S-1 is not inhibited. These results provide definitive demonstration that the product of a cloned S gene has S specific pollen inhibitory activity.