TRANSLATIONAL REGULATION OF CHLOROPLAST GENES - PROTEINS BINDING TO THE 5'-UNTRANSLATED REGIONS OF CHLOROPLAST MESSENGER-RNAS IN CHLAMYDOMONAS-REINHARDTII

We have examined the effects of illumination, carbon source, and level s of chloroplast protein synthesis on trans-acting proteins that bind to the leaders of five representative chloroplast mRNAs, The accumulat ion of these five chloroplast mRNAs and the proteins they en code were measured in cells grown under identical conditions, Extracts from all cell types examined contain a minimum set of six chloroplast 5'-untra nslated region (UTR)-binding proteins (81, 62, 56, 47, 38, and 15 kDa) , Fractionation results suggest that multiple forms of the 81-, 62-, a nd 47 kDa proteins may exist, A 36-kDa protein was found in all cells except those deficient in chloroplast protein synthesis, Binding of th e 81-, 47, and 38-kDa proteins to the rps12 leader is effectively comp eted by the atpB or rbcL 5'-UTRs, indicating that the same proteins bi nd to all three leaders. In contrast, these three proteins do not bind to the nuclear encoded alpha-1 tubulin leader, which bound novel prot eins of 110, 70, and 43 kDa. Cis-acting sequences within the 5'-UTRs o f two chloroplast mRNAs (rps7 and atpB) have been identified which are protected from digestion by RNase T1 by extracts enriched for the 81- , 47-, and 38-kDa proteins.