In Synechocystis sp. PCC 6803, the loop domain (aa 1-70) of the phycobilisome core-membrane linker, L-CM, was found to interact with the glycosyl transferase homolog, Sll1466. Growth of a Sll1466 knockout mutant was slightly faster in low light, but strongly inhibited in high light: the phenotype is discussed in relation to the regulation of light energy transfer to photosystem II. At the molecular level, the mutant shows the following changes compared to the wild type: (1) a smaller size and higher mobility of phycobilisomes on the thylakoid membrane, and (2) a changed lipid composition of the thylakoid membrane, especially decreased amounts of digalactosyl diacylglycerol. These results indicate a profound regulatory role for Sll1466 in regulating photosynthetic energy transfer. (C) 2011 Elsevier Inc. All rights reserved.