Molecular characterization of two clusters of genes encoding the Type I CAB polypeptides of PSII in Nicotiana plumbaginifolia

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Abstract
A Nicotiana plumbaginifolia genomic library in the phage Charon 34 was used to isolate and characterize 7 full-length genes and part of an 8th gene encoding chlorophyll a/b-binding (CAB) polypeptides. These genes are arranged in two clusters. All the genes within the clusters are arranged in opposite orientation to their neighbours. The nucleotide sequences of two genes, one from each cluster, show that both genes, designated Cab-E and Cab-C, encode very similar proteins (95.9% of homology) corresponding to type I photosystem II polypeptides. Southern blot analysis suggests that at least 19 CAB genes encoding type I PSII CAB polypeptides are present in the N. plumbaginifolia genome. We also describe the presence within the N. plumbaginifolia genome of CAB genes encoding PSII type II CAB polypeptides and PSI type I CAB polypeptides. The sequences of the 5′ flanking region of three different CAB genes (Cab-E, Cab-C, and CAB-F) were determined. Two of them (Cab-C and Cab-F) share extensive homology, whereas the Cab-E promoter shows homology to Cab-C and Cab-F only in a unique region extending from the CAAT box to the TATA box. This conserved sequence is also found in the same position in promoters of CAB genes encoding type I PSII polypeptides from other plant species.