Light-induced structural adaptation of the bundle-shaped phycobilisome from thylakoid-lacking cyanobacterium Gloeobacter violaceus.

Gloeobacter diverged from other lineages early in cyanobacterial evolution, preferentially growing under low light intensity conditions. Among cyanobacteria, G. violaceus exhibits unique features, including lack of a thylakoid membrane and bundle-shaped antenna phycobilisomes (PBSs), densely packed and well-organized on the plasma membrane. However, without high-resolution structures, it has remained unclear how G. violaceus PBSs assemble into a bundle-shaped configuration. Here we solve the cryo-EM structures of PBSs from G. violaceus cells cultured under low (Sr-PBS) or moderate (Lr-PBS) light intensity. These structures reveal two unique linker proteins, L and L, that play a key role in the PBS architecture. Analysis of the bilin arrangement indicates that the bundle-shaped structure allows efficient energy transfer among rods. Moreover, comparison between Lr-PBS and Sr-PBS uncovers a distinct mode of adaption to increased light intensity wherein the ApcA-ApcB-ApcD layer can be blocked from binding to the core by altering structural elements exclusively found in the G. violaceus L. This study illustrates previously unrecognized mechanisms of assembly and adaptation to varying light intensity in the bundle-shaped PBS of G. violaceus.