The transcriptional regulator GalR self-assembles to form highly regular tubular structures

The Gal repressor regulates transport and metabolism of D-galactose in Escherichia coli and can mediate DNA loop formation by forming a bridge between adjacent or distant sites. GalR forms insoluble aggregates at lower salt concentrations in vitro, which can be solubilized at higher salt concentrations. Here, we investigate the assembly and disassembly of GalR aggregates. We find that a sharp transition from aggregates to soluble species occurs between 200 and 400 mM NaCl, incompatible with a simple salting-in effect. The aggregates are highly ordered rod-like structures, highlighting a remarkable ability for organized self-assembly. Mutant studies reveal that aggregation is dependent on two separate interfaces of GalR. The highly ordered structures dissociate to smaller aggregates in the presence of D-galactose. We propose that these self-assembled structures may constitute galactose-tolerant polymers for chromosome compaction in stationary phase cells, in effect linking self-assembly with regulatory function.