Post‐transcriptional and post‐translational regulatory steps are crucial in controlling the appearance and stability of thylakoid polypeptides during the transition of etiolated tobacco seedlings to white light

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Abstract
We have investigated the expression of nuclear-encoded chloroplast proteins that are not associated with chlorophyll (the lumenal 33-kDa and 23-kDa polypeptides of the oxygen-evolving system of photosystem II, plastocyanin and the Rieske Fe/S protein) by comparing mRNA-accumulation rates with those of the corresponding proteins during illumination of etiolated tobacco seedlings. Using subcellular fractionation, pulse/chase, Northern and Western techniques, we found that the biogenesis and stability of these proteins are regulated both translationally, as well as post-translationally, including the efficiency of mRNA uptake into polysomes, processes that operate between translation and assembly or monitor the status (soluble and membrane-attached) of a terminally processed polypeptide. Polypeptide synthesis is generally not limited by mRNA amounts. For instance, steady-state transcript levels may increase 10-fold during illumination, while those associated with polysomes increase only 2-3-fold without measurable influence on the rate of protein synthesis. The 23-kDa and Rieske polypeptides are predominantly membrane associated, but plastocyanin and the 33-kDa polypeptide are distributed among both soluble and membrane-associated protein fractions. Plastocyanin appears to be comparably stable in both forms. However, for the 33-kDa polypeptide, only the membrane-attached form is stable (> 8 h) and only this pool increases upon illumination. Its soluble form is rapidly degraded with a half-life of approximately 1 h under the chosen conditions. Our findings probably reflect part of a more general regulatory principle operating in the differentiation and maintenance of subcellular structure.