Targets and functions of the chloroplast ribonucleoproteins CP33A and CP33B

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Chloroplast mRNAs show a tight coordination across various environmental conditions. This is not a function of transcriptional, but rather of post-transcriptional regulation. Strikingly, their half-lives are massively extended relative to their bacterial ancestry, a prerequisite for post-transcriptional control. We have recently characterized the essential chloroplast ribonucleoprotein CP33A that binds most chloroplast mRNAs and is required for their stabilization. The mechanism behind global chloroplast RNA stabilization by Cp33A remains unclear. By using a combination of RIP-Seq, iCLIP and reverse genetics, I propose to decode CP33A’s stunning ability to serve the entire chloroplast mRNA pool.A special case among the chloroplast transcriptome is the psbA mRNA, which codes for the D1 protein. D1 is at the core of photosystem II and has to be constantly produced in the light due to permanent photodamage. How D1 production is regulated is one of the key questions in the field of photosynthetic gene expression. The psbA mRNA is present in vast amounts, arguably the most abundant mRNA on our planet. How are the masses of psbA mRNA stabilized and is stability regulated? We have identified a protein ligand of psbA, CP33B, which captures 90% of the psbA message. In prior experiments, we found an unexpected preference of CP33B for full-length psbA mRNA versus psbA fragments and that loss of CP33B leads to a reduction in psbA message early in plant development, when the need for the production of the photosynthetic machinery is highest. I propose to study the peculiar, possibly cooperative mode of CP33B-psbA interaction and the role of CP33B in psbA stabilization, also in the light of expected redundancy with an evolutionary sister protein. This work will unravel the mechanisms of stabilization of this key RNA of photosynthesis that have not been accessible so far.