Some Regulatory Proteins Can Activate Translation

Positive regulation of translation is used to control protein synthesis in chloroplasts after light stimulation. Synthesis of many chloroplast proteins is induced as much as a hundred-fold by light. The levels of some of these proteins are controlled by transcription, others by translation and others by protein degradation.

For example, the mRNA for the large subunit of Rubisco accumulates in developing chloroplasts even in the dark. [Rubisco is ribulose bisphosphate carboxylase, a critical enzyme in the fixation of carbon dioxide during photosynthesis. It is the most abundant protein on earth.] Another example is PsbA (= D1 protein) a component of photosystem II. Translation of these mRNAs is controlled by proteins encoded by the nucleus that act as translational activators. These proteins bind to an adenine rich region in the 5'-UTR of the mRNA. The activators bind to the mRNA in the light and allow translation. In the dark, they do not bind to the mRNA which cannot be translated due to its unfavorable secondary structure (Fig. 11.06).

The translational activator cPABP (chloroplast polyadenylate binding protein) exists in two conformations, only one of which can bind RNA. The interconversion of the two forms of cPABP is controlled by light. Energized electrons from photosystem I are passed down a short electron transport chain to cPABP. The electrons reduce the disulfide form of cPABP to the sulfhydryl form. The reduced sulfhydryl form can bind to RNA and activate translation, whereas, the disulfide form cannot (Fig. 11.07).

cPABP (chloroplast polyadenylate binding protein) A translational activator protein that controls expression of chloroplast mRNA Rubisco (ribulose bisphosphate carboxylase) A critical enzyme in the fixation of carbon dioxide during photosynthesis translational activator A protein that binds to mRNA and promotes its translation

A) TRANSLATIONAL ACTIVITY IS LOW

Loop prevents interaction with ribosome

Loop prevents interaction with ribosome

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