Two anther-expressed genes downstream of SLG9:: identification of a novel S-linked gene specifically expressed in anthers at the uninucleate stage of Brassica campestris (syn. rapa) L.

The sporophytic self-incompatibility (SI) system in Brassica campestris (syn. rapa) is controlled by multiple alleles of a single locus, S, in which SLG and SRK are located. SLG and/or SRK appear to comprise the female component of the recognition reaction of SI. The gene encoding the male component must also be located at the S locus but has not been identified to date. RNA gel blot analysis with regions flanking SLG(9) and SRK9 revealed two anther-expressed genes 3' of the SLG(9) gene. Two cDNA clones were isolated using the 3'-flanking region of SLG(9) as probe. Because of sequence similarity with the Brassica SLL2 gene, one cDNA clone was designated SLL2-S-9 (for S Locus-Linked 2-S-9 gene). The 5'-region of SLL2-S-9 was not present in other SLL2 genes examined. The second gene was novel and was designated SAE1-S-9 (for S locus Anther Expressed 1-S-9 gene). SAE1-S-9 and SLL2-S-9 were located approximately 2.5 kb and 5.5 kb, respectively, downstream of SLG(9). A 1.4-kb transcript for SLL2-S9 was detected in both vegetative and reproductive tissues, whereas 1.9-kb and 2.4-kb transcripts were specifically detected in the anther of S-9 haplotype. SAE1-S-9 was specifically expressed in the anther at the uninucleate stage. This gene encoded a novel polypeptide of 257 amino acids. Database analysis revealed a homologous sequence in the upstream region of SRK3 of B. oleracea. We discuss possible relationships between SLL2-S-9 and SAE1-S-9 and SI.