Nucleoid Compaction Influences Carboxysome Localization and Dynamics in Synechococcus elongatus PCC 7942

The bacterial nucleoid is not just a genetic repository—it serves as a dynamic scaffold for spatially organizing key cellular components. ParA-family ATPases exploit this nucleoid matrix to position a wide range of cargos, yet how nucleoid compaction influences these positioning reactions remains poorly understood. We previously characterized the Maintenance of Carboxysome Distribution (Mcd) system in the cyanobacterium Synechococcus elongatus PCC 7942, where the ParA-like ATPase McdA binds the nucleoid and interacts with its partner protein, McdB, to generate dynamic gradients that distribute carboxysomes for optimal carbon fixation. Here, we investigate how nucleoid compaction impacts carboxysome positioning, particularly during metabolic dormancy when McdAB activity is downregulated. We demonstrate that a compacted nucleoid maintains carboxysome organization in the absence of active McdAB-driven positioning. This finding reveals that the nucleoid is not merely a passive substrate for positioning, but an active player in spatial organization. Given the widespread role of ParA-family ATPases in the positioning of diverse cellular cargos, our study suggests that nucleoid compaction state is a fundamental, yet underappreciated, determinant of mesoscale organization across bacteria.