Style self-incompatibility gene products of Nicotlana alata are ribonucleases

SELF-INCOMPATIBILITY in flowering plants is often controlled by a single nuclear gene (the S-gene) having several alleles 1 . This gene prevents fertilization by self-pollen or by pollen bearing either of the two S - alleles expressed in the style. Sequence analysis shows that three alleles of the S gene of Nicotiana alata encode style glycoproteins 2,3 with regions of defined homology. Two of the homologous regions also show precise homology with ribonucleases T 2 (ref. 4) and Rh (ref. 5). We report here that glycoproteins corresponding to the S 1 , S 2 , S 3 , S 6 and S 7 alleles isolated from style extracts of N. alata 6 are ribonucleases. These style S -gene-encoded glycoproteins account for most of the ribonuclease activity recovered from style extracts. The ribonuclease specific activity of style extracts of the self-incompatible species N. alata is 100–1,000-fold higher than that of the related self-compatible species N. tabacum . These observations implicate ribonuclease activity in the mechanism of gametophytic self-incompatibility.