A gene required for the regulation of photosynthetic light harvesting in the cyanobacterium Synechocystis PCC6803

In cyanobacteria, state transitions serve to regulate the distribution of excitation energy delivered to the two photosystem reaction centres from the accessory light harvesting system, the phycobilisome. The trigger for state transitions is the redox state of the cytochrome bf complex/plastoquinone pool. The signal transduction events that connect this redox signal to changes in light harvesting are unknown. In order to identify signal transduction factors required for the state transition, random cartridge mutagenesis was employed in the cyanobacterium Synechocystis PCC6803 to generate a library of random, genetically tagged mutants. The state transition in cyanobacteria is accompanied by a change in fluorescence emission from PS2. By using a fluorescence video imaging system to observe this fluorescence change in mutant colonies it was possible to isolate mutants unable to perform state transitions. Five such mutants were isolated and in all cases disruption of the gene slll926 found to account for the phenotype. slll926 has only one significant homologue in current databases, a gene in Anabaena PCC7120. A hydropathy analysis would suggest that both proteins are membrane associated. An insertional inactivation mutant of slll926 (δslll926) is specifically unable to perform state transitions: no impairments in electron transport, in light harvesting, or in reaction centre/phycobilisome assembly or function are evident. The product of slll926 may therefore be a signal transduction factor. slll926 inactivation in mutants lacking photosystem 1 or photosystem 2 leads to changes in phycobilisome coupling with the remaining photosystem. This strongly supports a mobile phycobilisome model for state transitions and suggests that state transitions are brought about by changes in the affinity of the phycobilisomes for both photosystems. δslll926 has a doubling time similar to the wild-type at all but very low light intensities. This implies that a major physiological role of state transitions is to maximise the efficiency of light utilisation