Products of Protein Synthesis by Isolated Pea Etioplasts

Analysis of the products of etioplast protein synthesis by SDS polyacrylamide gel electrophoresis reveals a pattern of labeled peaks (Fig. 12) very similar to the pattern produced by isolated chloroplasts (Fig. 3) peaks A to F can be recognized, and all are removed by digestion with pronase. The most highly labeled peak, B, runs coincident with the large subunit of Fraction I protein and is released in soluble form when etioplasts are lysed in hypotonic buffer and centrifuged at 105,000g for 1 h. The other labeled peaks sediment with the membrane fraction. We conclude from these results that etioplasts probably make the same range of proteins as do chloroplasts. 

One difference that we found between protein synthesis by etioplasts and that by chloroplasts is in the relative labeling of peaks B and D. In chloroplasts, these peaks contain similar amounts of radioactivity in most experiments, with peak B sometimes being slightly more labeled than peak D. However, in etioplasts peak B is by much the most highly labeled component. When etiolated pea plants are greened under continuous light over 4 days, the products of protein synthesis by plastids—isolated at inter- 

A method for isolating chloropiast envelopes was first devised by Mackender and Leech. We used a slight modification of this method by including a density gradient step introduced by Poincelot to isolate en- 

These results are interpreted to mean that at least one of the chloroplast envelope proteins is synthesized by chloroplast ribosomes. This conclusion is supported by the result of an in vivo experiment in which detached pea shoots were fed with P S]methionine, with and without cycloheximide at 2 Incorporation of label into PSII protein was in- 

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