Photoinhibition, or cell damage caused by excessively intense light is a major issue for the industrial use of cyanobacteria. To investigate the mechanism of responses to extreme high light intensity, gene expression analysis was performed using the model cyanobacterium Synechocystis sp. PCC 6803 (PCC 6803) cultured under various light intensities. The culture profile data demonstrated that, in contrast to the slow cell growth observed under low light intensities (30 and 50 mol m(-2) s(-1)), maximal cell growth was observed under mid light conditions (300 and 1000 mu mol m(-2) s(-1)). PCC 6803 cells exhibited photoinhibition when cultured under excessive high light intensities of 1100 and 1300 mu mol M-2 s(-1). From the low to the mid light conditions, the expression of genes related to light harvesting systems was repressed, whereas that of CO2 fixation and of Dl protein turnover-related genes was induced. Gene expression data also revealed that the down-regulation of genes related to flagellum synthesis (pilA2), pyridine nucleotide transhydrogenase (pntA and pntB), and sigma factor (sigA and sigF) represents the key responses of PCC 6803 under excessive high light conditions. The results obtained in this study provide further understanding of high light tolerance mechanisms and should help to improve the productivity of bioprocess using cyanobacteria. (C) 2018, The Society for Biotechnology, Japan. All rights reserved.