Left: Synechococcus elongatus cells bearing one or multiple carboxysomes (green pentagons). McdB proteins on the surface of the carboxysomes create a gradient of McdA (pink) that oscillates across the surface of the nucleoid. Carboxysomes move to the highest concentration of McdA, but McdB causes McdA to dissociate more easily from the nucleoid surface. In cells containing one carboxysome (top), the carboxysome sits at the McdA-depleted region of the nucleoid. In cells containing two or more carboxysomes, the carboxysomes move apart from each other until they end up equally spaced across the nucleoid. Right: Schematic diagram of a carboxysome (adapted from http://2014.igem.org/Team:Bielefeld-CeBiTec/Project/CO2-fixation/Carboxysome; CC BY 3.0). Carboxysomes contain the key enzymes for photosynthesis – RuBisCO and carbonic anhydrase. McdB proteins (light green diamonds) on the surface of carboxysomes allow the carboxysomes to interact with McdA on the surface of nucleoids.