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 solved the cryo-EM structures of PBSs from G. violaceus cells cultured under low (Sr-PBS) or moderate (Lr-PBS) light intensity. These structures revealed two unique linker proteins, L _RC ^91kDa and L _RC ^81kDa , that play a key role in the PBS architecture. Analysis of the bilin arrangement indicated that the bundle-shaped structure allows efficient energy transfer among rods. Moreover, comparison between Lr-PBS and Sr-PBS uncovered a distinct mode of adaption to increased light intensity wherein the ApcA ₂ -ApcB ₃ -ApcD layer can be blocked from binding to the core by altering structural elements exclusively found in the G. violaceus L _CM . This study illustrates previously unrecognized mechanisms of assembly and adaptation to varying light intensity in the bundle-shaped PBS of G. violaceus .