Sequence of two tomato nuclear genes encoding chlorophyll a/b-binding proteins of CP24, a PSII antenna component

Abstract

We have isolated and characterized (Fig. 1) two tomato nuclear genes each encoding a protein of 256 residues with discernible sequence identity with other CAB polypeptides. The two genes, which we designate Cab 10A and Cab 10B (tomato genes designated Cab l through Cab9 encoded other CAB polypeptides of PSI and PSII [9, 10]), are tandemly linked and are separated by approximately 5 kb. Each gene contains a single, short intron 199 bp in CablOA, 97 bp in CablOB whose position (same in both genes) was determined by comparisons with cDNA clones. An intron in an equivalent position is also found in the tomato PSI Type II and Type III CAB genes [9]. Within the coding region, CablOA and Cab 10B are 92.5 ~o identical, but the introns and 5' and 3' flanking regions show little similarity. Comparison of the sequence of the proteins encoded by Cab 10A and Cab 10B with the N-terminal sequence of the homologous (see below) spinach protein (Fig. 2A) indicates that the transit peptide of the CAB10A protein consists of 48 residues, and the transit peptide of CAB 10B consists of 46 residues. Thus, the mature CAB10A protein has 208 residues, with a calculated molecular weight of 22.6 kDa, and the mature CAB10B protein has 210 residues, with a calculated molecular weight of 22.8 kDa. CAB 10A and CAB10B are 94.5~o identical within the mature part of the protein, and the sequences of the transit peptides are less similar. Three hydrophobic regions are found in all CAB polypeptides examined to date, and it has therefore been hypothesized that they represent alpha-helices which span the thylakoid membrane, and that the three-dimensional structure of all CAB polypeptide is therefore similar despite the high level of primary sequence divergence [3, 10]. The hydropathy plots of CAB10A and CAB10B also indicate 3 hydrophobic regions of sufficient length to traverse the thylakoid membrane (Fig. 2B). It should be noted, however, that algorithms to predict secondary structure (e.g. [4]) do not always indicate complete overlap between the hydrophobic regions in the CAB polypeptides and regions predicted to assume the alpha-helix conformation, and this is especially true with CAB 10A and CAB 10B, where no extensive alpha-helix conformation is predicted (not shown) anywhere between the first and third hydrophobic regions. (However, these programs can predict alpha-helical regions in only 50 ~o -60 ~o of the cases where they indeed occur, as determined with proteins whose structure has been experimentally determined [4]). In addition to possessing the three hydrophobic regions, the CAB10A and CAB10B proteins also share the two regions of sequence similarity (Fig. 2C) found in all CAB proteins [10]. The two conserved regions include the first and third hydrophobic regions and their immediate N-terminal regions, and in these regions CAB 10A and CAB 10B dis-