An intronic element directs endosperm-specific expression of the sbellb gene during barley seed development

The sbeIIa and sbeIIb genes, both, encoding starch branching enzymes, are differentially transcribed during barley seed development. The sbeIIb gene is expressed exclusively in the endosperm, while the sbeIIa gene is also expressed in embryonic and vegetative tissues. Sequence analysis revealed that promoter elements are not responsible for the tissue-specific expression of sbeIIb. Instead, it was found that the second sbeIIb intron harbors, a sequence with a high degree of identity to the B-box present in the potato patatin promoter. Electrophoresis mobility shift assays suggested that nuclear proteins from leaves but not from endosperm bind to the intronic B-box-like (Bbl) element. In the presence of this second intron, the sbeIIb promoter could not drive transcription of green fluorescent protein (g/p) reporter constructs in transgenic embryo cells. However, when the intron was deleted, the gfp gene was transcribed. Deletion of the Bbl element also resulted in gfp expression, although not to the same level as for the intron-less construct. The results suggest that the intronic Bbl element participates in directing endosperm-specific expression of sbeIIb by recruiting negative transcription factors in non-expressing tissues.