AbstractsBiology & Animal Science

In photosynthetic organisms, control of light-harvesting is a key component of acclimation mechanisms that optimize photon conversion efficiencies. In this thesis, the interrelation of short- and long-term regulation of light-harvesting at photosystem II (PSII) was analyzed in the green alga Chlamydomonas reinhardtii. This model organism is able to gain carbon and energy through photosynthetic carbon dioxide fixation as well as heterotrophic feeding. A lowered inorganic or increased organic carbon supply reduces the rate of NADPH consumption by the Calvin cycle, resulting in an over-reduced photosynthetic electron transport chain and increased excitation pressure at photosystem II. A combination of molecular biology, biochemistry, chlorophyll fluorescence and physiological analyses revealed that a reduction in functional antenna size efficiently relieved excitation pressure on PSII under these conditions. Particularly, translation control on PSII-associated major light-harvesting proteins (LHCII) replaced state transitions as an initial protection mechanism in the long term. The LHCII translation repressor NAB1 emerged as key factor implicated in the acclimation to the prevailing carbon assimilation mode. The level of NAB1 was increased under carbon dioxide limitation, and expression control based on modulated promoter activity. Application of a photosynthetic electron transport inhibitor and a perturbed NAB1 accumulation in a state transition mutant suggested that chloroplast retrograde signals control nuclear NAB1 expression. To further investigate this retrograde signaling, a reporter system was developed that enables detailed promoter analyses. Systematic truncation studies identified a promoter fragment of 152 bases, which comprised essential regulatory elements and can be used as tool for the identification of cis-regulatory elements in future studies. Furthermore, chloroplast redox poise was shown to modulate the extent of LHCII translation repression in the cytosol via cysteine based redox control of NAB1. In response to moderate light intensity changes, a fine-tuning system comprising specific single cysteine nitrosylation and thioredoxin mediated re-reduction adjusted NAB1 activity to the demand for light-harvesting antenna proteins. This is the first mechanistic description of redox based translation control of nuclear encoded photosynthesis associated genes. Overall, this thesis describes regulatory circuits that adjust light-harvesting capacity over a range of time scales, involving nuclear and cytosolic expression control as well as short-term responses in the chloroplast, and provides new insights into interorganellar communication that ensures optimal photon capture.