EXPRESSION OF GENES ENCODING THE TOBACCO CHLOROPLAST PHOSPHATE TRANSLOCATOR IS NOT LIGHT-REGULATED AND IS REPRESSED BY SUCROSE

A cDNA encoding the complete precursor of the phosphate translocator o f the chloroplast inner envelope membrane has been isolated from a tob acco leaf (Nicotiana tabacum cv: Samsun) lambda gt 11 library. The tob acco cDNA is 1546 bp in length and encodes a precursor protein of 401 amino acid residues with a deduced molecular weight of 43705. A putati ve processing site between Ala-73 and Ala-74 of the precursor protein is suggested by comparison with the N-terminal sequences of the pea an d spinach proteins. Removal of the transit peptide produces the mature protein of 328 amino acid residues with a molecular weight of 36038. Southern blot analysis suggests there is probably one copy of the phos phate translocator gene in the pea haploid genome and two copies in th e tobacco haploid genome, one derived from each ancestral parental gen ome. Messenger RNAs essentially equivalent in size to the cDNAs (appro x. 1.6 kb) were detected in extracts of all organs examined from tobac co and pea, including leaves, stems, sepals, petals, seed-pods, tendri ls and roots. An immunochemically related protein of a similar size to the phosphate translocator was detected in the equivalent pea organs. The levels of both mRNA and protein in non-photosynthetic organs were lower than those in photosynthetic organs. Tobacco phosphate transloc ator mRNA was present at high levels in etiolated tissue and did not i ncrease significantly after 24 h illumination. Germination and growth of tobacco seedlings in the presence of sucrose caused a 3.3-fold decr ease in the level of the phoshate translocator mRNA.