Cloning and nucleotide sequence of two S-RNases from Lycopersicon peruvianum (L.) Mill.

Self-incompatibility is one of the mechanisms that has evolved to encourage outbreeding in flowering plants (de Nettancourt, 1977). One particular type of self-incompatibility, gametophytic self-incompatibility, is found in members of Solanaceae, such as the wild tomato, Lycopersicon peruvianum (Mau et al., 1986). Gametophytic self-incompatibility in L. peruvianum is controlled by a single locus (S-locus) with many alleles (Tanskley and Loaiza-Figueroa, 1985; Rivers et al., 1993). Incompatible pollen grains genninate normally, but at some point during growth through the style toward the ovaries, pollen tube growth is arrested (Newbigin et al., 1993). A class of glycoproteins with ribonuclease activity (SRNases) are products of the S-locus in the style (McClure et al., 1989); the genes that encode these proteins co-segregate with alleles of the S-locus (Anderson et al., 1989; Rivers et al., 1993). As part of our study of variability at the S-locus, we have cloned a number of S-RNase alleles from natural populations of L. peruvianum. Here, we report the cloning and sequencing of SJand S7-RNases (Table I). The co-segregation of the S3and S7-RNases with alleles of the S-locus is reported elsewhere (Mau et al., 1986; Kowyama et al., 1994). The derived amino acid sequences from the N terminus of the SBor S7RNases correspond exactly with that obtained by direct sequencing of the purified protein (Mau et al., 1986; C. Kunz and M. Anderson, unpublished results). The S7-RNase has ribonuclease activity (Kowyama et al., 1993), but this has not been demonstrated for the S3-RNase. The sequence of the S3-RNase is 98% identical with the previously characterized S5 allele from L. peruvianum accession No. LA2163 (Rivers et al., 1993). To our knowledge, this is the highest degree of similarity yet found between two SRNase alleles.