ApcD is required for state transition but not involved in blue-light induced quenching in the cyanobacterium Anabaena sp PCC7120

Pbycobilisomes (PBS) are able to transfer absorbed energy to photosystem I and II, and the distribution of light energy between two photosystems is regulated by state transitions. In this study we show that energy transfer from PBS to photosystem I (PSI) requires ApcD. Cells were unable to perform state transitions in the absence of ApcD. The apcD mutant grows more slowly in light mainly absorbed by PBS, indicating that ApcD-dependent energy transfer to PSI is required for optimal growth under this condition. The apcD mutant showed normal blue-light induced quenching, suggesting that ApcD is not required for this process and state transitions are independent of blue-light induced quenching. Under nitrogen fixing condition, the growth rates of the wild type and the mutant were the same, indicating that energy transfer from PBS to PSI in heterocysts was not required for nitrogen fixation.