Chlorophyll a/b-Binding Protein Gene Expression in Cotton

To investigate the regulation of genes encoding CAB proteins in cotton (Gossypium hirsutum) and identify potentially useful promoters, we determined the structure and expression of two genes (Lhcbl*Z and Lhcb3*l) encoding type I and type 111 proteins of the LHCII subfamily (Table I). Northern analyses of the gene expression pattem in cotton demonstrated substantial LHCII transcript accumulation in the following photosynthetic organs and tissues: leaves (greatest accumulation), bracts, seedling stems, pericarp, and immature petals (least accumulation). No LHCII transcripts were detected in roots, mature petals, seeds, anthers, ovaries and pistils, or immature fiber and ovules. Although the pattern of expression was the same with either an Lhcbl or an Lhcb3 gene probe, differences in the intensity of hybridization suggested that Lhcbl transcripts may be as much as 10-fold more abundant than Lhcb3 transcripts. To test the leve1 of expression conferred by the LhcbZ*l gene promoter, we linked its upstream and downstream regions (approximately 700 and 500 bp of sequence, respectively) to the GUS gene (Jefferson, 1987). In transient expression assays performed with cotton cotyledons (Hamilton et al., 1992), this construction exhibited only 5-fold less activity than a very highly expressed GUS construction containing a double CaMV 35s enhancer sequence and the tobacco mosaic virus omega sequence (Skuzeski et al., 1990). The relatively high expression conferred by the LhcbZ*l gene promoter may be due in part to three GATA motifs that reside between the TATA and CAAT box sequences. These GATA motifs are highly conserved in Lhcbl gene promoters from severa1 different plants (reviewed in Castresana et al., 1987) and are also found in the CaMV 35s promoter (Lam and Chua, 1989).