The S-specificity determinant in pollen is an S-locus F-box protein, named SLF or SFB (Kao and Tsukamoto, 2004). SLF genes were identified by sequenc ing genomic DNA adjacent to the S-RNase gene. Antirrhinum AhSLF2 was the first SLF gene to be identified. It is located only 9 kb from S2-RNase (Lai et al. 2002). S-RNase-linked F-box genes were identified independently in the Rosaceae; these were named SFB (S-locus F-box) (Ushijima et al. 2003). Sequence analyses of SFB genes provide evidence of positive selection, strongly supporting their role as recognition proteins (Ikeda et al. 2004).
Several key properties of SLF/SFB proteins are consistent with them being pollen-S. SLF genes are expressed in mature pollen and exhibit sequence polymorphism (Ushijima et al. 2003; Ikeda et al. 2004). A histidine-tagged AhSLF2 protein and the yeast-two-hybrid system were used to demonstrate interaction between the C-terminal region of AhSLF2 and S-RNase (Qiao et al. 2004), although binding was not S-specific. Together, these data support identification of SLF/SFB as the pollen S-determinant.
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