Translation of mRNAs for subunits of chloroplast coupling factor 1 in spinach.
The chloroplast coupling factor 1 consists of five nonidentical subunits, three of which (alpha, beta, and epsilon subunits) have been shown in several laboratories to be synthesized within chloroplasts. The site of synthesis of the remaining two (gamma and delta subunits) was investigated by analyzing products directed by spinach leaf RNAs in wheat germ and reticulocyte translation systems in vitro. It was found that poly(A)(+) RNA directs the synthesis of two distinct polypeptides, one of which is immunochemically related to the gamma subunit but is 4,000 daltons larger. The other shares antigenic sites with the delta subunit but is 8,000 daltons larger. When wheat germ or reticulocyte translation systems were programmed with RNAs from purified chloroplasts, the only products related to CF(1) that we could detect were a putative precursor of beta, 2,000 daltons larger than the mature subunit, and some smaller polypeptides, which appear to be incomplete translation products of beta. From these results it appears likely that the gamma and delta subunits are synthesized in the cytoplasm as larger precursors and that beta is synthesized within the chloroplast as a precursor.
Waksman Institute of Microbiology, Rutgers University, Piscataway, New Jersey 08854.
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