Oligomeric structure of the repressor of the bacteriophage Mu early operon

The regulation of the lytic and lysogenic development in the life cycle of bacteriophage Mu is regulated in part by its repressor, c, which binds to three operator sites, O1, O2 and O3, overlapping two divergent promoters. The oligomeric structure of this repressor protein was investigated by hydrodynamic and biochemical methods. Size-exclusion chromatography, analytical ultracentrifugation, dynamic light scattering, crosslinking and direct electron microscopy observations suggest that c exists primarily as a hexamer with a molecular mass of 120-140 kDa at low concentrations, i.e. in the 10-microM range. This molecule undergoes a self-assembly process leading to dodecamers and higher order species as the concentration is further increased in a manner depending on the nature of the solvent. Our results also suggest that these species have an elongated structure, and a possible arrangement of the subunits within the hexamer is proposed. The implication of this unusual quaternary structure for a repressor in its interaction with the operator sites O1 and O2 remains to be elucidated.