Analysis of three tissue-specific elements from the wheat Cab-1 enhancer.

The genes encoding the major light-harvesting antenna chlorophyll a/b-binding protein (Cab) of higher plants are regulated by light at the transcriptional level. In addition, their expression is largely restricted to photosynthetically competent organs such as leaves. A 268 bp fragment of the Cab-1 promoter from wheat functions as a light-responsive and organ-specific enhancer in transgenic tobacco. Using DNase I footprinting, four different regions have been mapped (Cab1-A, Cab1-B, Cab1-C and Cab1-D) in this fragment that bind to protein factors in tobacco nuclear extracts. Three of these regions (A, B and C) coincide with sequences that have been found to be functionally important from previous cis-element analyses. Synthetic tetramers of these three sites interact with different proteins in gel retardation assays. In addition, cross-competition analyses demonstrate that Cab1-C is likely to interact with ASF-2, a tobacco DNA-binding activity that binds to a conserved GATA element found in many dicot Cab promoters. In transgenic tobacco, a 95 bp fragment of the Cab-1 enhancer containing the A, B and C regions can confer leaf expression when fused upstream of a truncated derivative of the cauliflower mosaic virus (CaMV) 35S promoter. However, expression observed with this enhancer fragment in the promoter context of these studies does not appear to be significantly dependent on light. Similar results were obtained with synthetic tetramers of Cab1-A, -B or -C. These data thus suggest that the wheat Cab-1 enhancer contains at least three distinct elements that contribute to leaf-specific expression in transgenic tobacco. Interaction between factors binding to these positive elements and those that bind elsewhere in the Cab-1 enhancer may be necessary for light-responsive transcriptional activation.