(i) Adaptation to low iron availability

We are currently investigating the adaptation and remodeling of the photosynthetic apparatus as a response to iron-deficiency. We are also interested in an understanding of chloroplast iron-homeostasis as a whole. To gain insights into chloroplast iron-homeostasis we are carrying out a comparative proteomic approach and take advantage of stable isotope labeling by amino acids in Chlamydomonas (Naumann et al., 2007; Naumann et al., 2005) The investigation of putative candidate gene products in plastid iron homeostasis is done by reverse genetics using an RNAi approach as described by Cerutti and colleagues (Rohr et al., 2004). This approach was already successfully employed in our laboratory to suppress expression levels of Lhca3 (Naumann et al., 2005) and ferritin (Busch et al., 2008). Momentarily a focus is on the analysis of PGRL1 in iron-deficiency responses. In an RNAi strain, which accumulates lower PGRL1 levels in both Fe –replete and –starved conditions, the photosynthetic electron transfer rate is decreased, respiratory capacity in Fe sufficient conditions is increased, and the efficiency of cyclic electron transfer (CEF) under iron-deprivation is diminished. Furthermore, pgrl1-kd cells exhibit iron-deficiency symptoms at higher Fe concentrations than does the wild-type, although the cells are not more depleted in cellular iron relative to the wild-type as measured by mass spectrometry. Thiol trapping experiments indicate iron-dependent and redox-induced conformational changes in PGRL1 that may interconnect iron metabolism and CEF and thereby partitioning between linear photosynthetic electron transfer and CEF. We propose therefore that PGRL1 in C. reinhardtii possesses a dual function in iron sensing as well as modulation of cyclic electron transfer (Petroutsos et al., 2009). Future experiments will be directed to elucidate protein-protein-interactions of PGRL1 and metal-binding dynamics of the protein.

Naumann, B., Busch, A., Allmer, J., Ostendorf, E., Zeller, M., Kirchhoff, H. and Hippler, M. (2007)
Comparative quantitative proteomics to investigate the remodeling of bioenergetic pathways under iron deficiency in Chlamydomonas reinhardtii.
Proteomics, 7, 3964-3979.

Naumann, B., Stauber, E.J., Busch, A., Sommer, F. and Hippler, M. (2005)
N-terminal processing of Lhca3 Is a key step in remodeling of the photosystem I-light-harvesting complex under iron deficiency in Chlamydomonas reinhardtii.
J Biol Chem, 280, 20431-20441.

Rohr, J., Sarkar, N., Balenger, S., Jeong, B.R. and Cerutti, H. (2004)
Tandem inverted repeat system for selection of effective transgenic RNAi strains in Chlamydomonas.
Plant J, 40, 611-621.

Busch, A., Rimbauld, B., Naumann, B., Rensch, S. and Hippler, M. (2008)
Ferritin is required for rapid remodeling of the photosynthetic apparatus and minimizes photo-oxidative stress in response to iron availability in Chlamydomonas reinhardtii.
Plant J, 55, 201-211.

Dimitris Petroutsos, Aimee M. Terauchi, Andreas Busch, Ingrid Hirschmann, Sabeeha S. Merchant, Giovanni Finazzi, Michael Hippler (2009)
PGRL1 participates in iron-induced remodeling of the photosynthetic apparatus and in energy metabolism in Chlamydomonas reinhardtii
J Biol Chem., 284(47), 32770-81