Exploring Position-Dependent Effects of pduM on Microcompartment Assembly and Catalysis

Bacterial microcompartments (MCPs) are protein-based organelles that trap metabolic enzymes inside a selectively permeable shell. PduM is an accessory protein of the Salmonella enterica Pdu MCP that lacks canonical shell domains and is important for proper MCP assembly. Here we explore the relationship between operonic position and copy number of pduM and Pdu MCP assembly and function. Transfer of pduM to an early operonic position resulted in mixed polyhedral and tubular compartments and reduced activity, while later insertions yielded almost wild-type polyhedra. Increased copy number of pduM restored polyhedral morphology but eliminated catalytic activity, decoupling structure from function. The tubular variant also showed chained cells, which suggests that a change in the shell geometry can block cytokinesis. Although PduM had a low predicted intrinsic disorder, it was strongly dosage-sensitive and tolerated most repositioning. The results suggest that operon architecture, gene position, and copy number are associated with the functional assembly of MCPs, but the relative contributions of local dosage, polar effects, and positional interactions are yet to be resolved.