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Light Effects on the Synthesis of Ribulose-1,5-Bisphosphate Carboxylase in Lemna gibba L. G-3 ¹

Biology Department and Molecular Biology Institute, University of California, Los Angeles, California 90024

Placing light-grown Lemna gibba L. G-3 into the dark results in a changed pattern of protein synthesis. Although the amount of protein in the tissue and the over-all rate of incorporation of [³⁵S]methionine into protein does not significantly decline during four days of darkness, the rate of synthesis of three polypeptides declines dramatically. One of these polypeptides is the chlorophyll a/b-binding protein and the two others are the large and small subunits of ribulose-1,5-bisphosphate carboxylase. The changed rates of synthesis of the two subunits were examined after transitions of plants from light to dark and dark to light. The in vivo synthesis of both subunits, while declining to a low level during four days of darkness, increases rapidly upon returning the plants to white light. In addition, the level of poly(A) mRNA coding for the precursor polypeptide of the small subunit of the enzyme falls to a low level in the dark and increases rapidly in response to white light. The increase in translatable mRNA for the small subunit is rapid enough to account for a major part of the increased synthesis of this subunit.