Regulation of Photosynthetic Electron Transport Genes in Chlamydomonas Chloroplasts

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0091020 David SternChloroplasts, the site of photosynthesis, are derived from a cyanobacterial ancestor and have progressively transferred most of their genetic information to the nucleus. Because chloroplasts contain up to 5,000 proteins, today the vast majority are of nuclear origin. Among these proteins are a myriad which are required for correct expression of the genes which remain in the chloroplast. This project focuses on nucleus encoded proteins which regulate messenger RNA lifetime in the green alga Chlamydomonas reinhardtii, as well as on how chloroplast ribosomes recognize the site at which protein synthesis must be initiated. This project will further earlier analyses which revealed that at least 5 nuclear genes encode products which affect the stability of petD mRNA, which encodes subunit IV of the cytochrome b6/f complex, an essential protein for photosynthesis. One of these genes, Mcd1, has been cloned and the details of its interaction with the petD mRNA will be elucidated. Another gene whose product may form part of a multiprotein complex with Mcd1 will also be studied. This gene, mcd4, was identified through a genetic screen based on photosynthetic capability. Mcd4 has been targeted because it appears to have unique attributes with respect to other nuclear genes involved in chloroplast biogenesis. The second aspect of the project concerns translation initiation, and in particular the interaction between the initiator tRNA and the nucleotide upstream of the translation initiation codon of petA, which encodes cytochrome f. The generality of this interaction will be investigated, as well as the role of the so-called Shine-Dalgarno sequence, whose functionality in bacterial protein synthesis is well-established, but whose role in chloroplasts is somewhat controversial and clearly variable. Overall, this project will elucidate mechanisms of communication between different compartments of eukaryotic cells, a fundamental issue in understanding intracellular regulation.