Isolation and characterization of a gene encoding a chlorophyll a/b-binding protein from mustard and the targeting of the encoded protein to the thylakoid membrane of pea chloroplasts in vitro.

Three independent clones carrying a mustard gene coding for the chlorophyll a/b-binding protein were isolated by screening a genomic library of mustard with a heterologous cDNA probe from pea. All of them encode the same CAB gene, which, as shown by sequence analysis and comparison with published CAB sequences, belongs to the family of type I PSII CAB genes, encoding a precursor protein of 266 amino acids. Several conserved sequence motifs are observed in the 5' and 3' non-coding region of the gene. The putative transcription start site could be localized to 60 bp upstream of SA-CAB1 initiator codon by S1 mapping. Plasmids were constructed which allow in vitro transcription and translation of the whole chlorophyll a/b-binding protein and of truncated species which lack increasing portions of the C-terminus. Whereas the in vitro import into pea chloroplasts is not affected by these C-terminal deletions, targeting to the thylakoid membrane is abolished by the removal of the C-terminal helical domain. Accordingly, the 54 amino acids which contain the C-terminal membrane-spanning helix and flanking regions is an essential component of the thylakoid targeting signal.