| The photosynthetic apparatus is an intricate multi-complex system, capable of responding to short-term and long-term environmental changes. Assembly of the photosynthetic system and its adaptation to constantly changing light conditions are subject to tight regulation, requiring signals exchange and coordination of expression of two genomes residing in two separate subcellular compartments, the chloroplast and the nucleus. This coordinate expression of photosynthetic proteins is primarily achieved by the transcriptional activation of nuclear genes and the translational activation of plastid genes. D1, a core protein of photosystem II encoded by psbA plastid gene shows the highest induction by light, 50 to 100-fold. At least two light signals act in control of translation. The first, termed "priming", is activated upon transition from dark to light, and switches on a basal rate of translation of chloroplast mRNAs. The second signal is functional during the light period and coordinates translation rate with light intensity. Both signals emanate from the photosynthetic light reactions. Whereas the second signal is though to be transduced by the ferredoxin-thioredoxin system, the identity or the nature of the signaling factors transducing the priming signal and controlling translation is yet unknown. By creating a novel screen, we were able to isolate mutants with increased rate of D1 translating in the dark. In order to identify and isolate the regulatory genes controlling the priming the map-based cloning technique is used. Furthermore, significant changes in expression of several light harvesting proteins RNA as well as changes in assembly of photosynthetic complexes in the dark were found in these mutants. We believe that further study of these mutants could contribute greatly to better understanding of the light regulation of the chloroplast genes translation.
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