[Life Sciences Seminar]
           
             
         ▶Subject: Chloroplast Signaling and the Secrets of Plastid Genes
           
         ▶Speaker: Prof. Jean-David Rochaix

                         (Departments of Molecular Biology and Plant Biology, University of Geneva)
                   
         ▶Date: 11:00AM/Sept./18(Fri.)/2015
            
         ▶Place: Life Science Bldg. #207
           
                 *Abctract
              A major challenge for plants is to adapt constantly to changes in environmental conditions, in particular to fluctuating light irradiance. Major advances have been reached in understanding this process which involves a large number of signaling components, among which a protein quartet plays a central role. It consists of two chloroplast protein kinases, Stt7/STN7 and Stl1/STN8 and two phosphatases, PPH1/TAP38 and PBCP which act mainly on the LHCII proteins and on the PSII core proteins. In this plastid signaling network the Stt7/STN7 kinase is especially important as it is involved both in state transitions, a process which allows for the balancing of the absorbed light excitation energy between PSII and PSI and in retrograde signaling which impacts nuclear gene expression. However chloroplast signaling may also affect chloroplast gene expression. Most chloroplast genomes contain between 100 and 200 genes which are involved in photosynthesis, plastid gene expression and protein synthesis, and in various metabolic functions. Moreover, several chloroplast open reading frames of unknown function have been identified.  Some of these genes, in particular those required for photosynthesis,  have non-essential functions while others, such as those of the plastid protein synthesizing system, appear to be essential under any growth condition.  To better understand the role of some of these essential genes, we have developed a repressible chloroplast gene expression based on the nuclear Nac2 gene driven by the vitamin-sensitive MetE and Thi4 riboswitch in the green unicellular alga Chlamydomonas reinhardtii. Nac2 encodes a protein targeted to the chloroplast which specifically acts on the psbD 5’UTR and is required for the accumulation of psbD mRNA. This system can be used for repressing any chloroplast  protein gene provided its coding sequence is fused to the psbD 5’UTR.  Using this strategy we have been able to conditionally deplete Chlamydomonas cells of RpoA, the ? subunit of the chloroplast RNA polymerase (1), Rps12, a plastid ribosomal protein (1), ClpP1, the catalytic subunit of the ATP-dependent ClpP protease (2) and ORF1995, a large chloroplast gene which appears to be involved both in  chloroplast protein import and in plastid gene expression. Taken together this analysis has revealed novel aspects of chloroplast gene expression and signaling, in particular a regulatory negative feedback circuitry, a chloroplast-triggered autophagic response as well as a plastid protein unfolded response.
1)Ramundo et al. (2013) Plant Cell 25, 167-186.
2)Ramundo at al. (2014) Plant Cell 26, 2201-2222.

                 
        ▶Inquiry: Prof. Youngsook Lee (279-2296) 
    
           * This seminar will be given in English.
       please refrain from taking photos during seminars. *