Bacteriophage T4 regA protein binds RNA as a monomer, overcoming dimer interactions

The stoichiometry of the complex formed between the T4 translational repressor protein regA and the 16 nt gene 44 recognition element (gene 44RE) RNA has been determined. Under quantitative binding conditions, the association of wild-type regA protein with gene 44RE RNA exhibits saturation at a 1:1 ratio of protein to RNA. It is known that regA protein exists as a dimer in protein crystals. Thus, the stoichiometry may be indicative of a regA dimer bound to two RNAs or a regA monomer bound to one RNA. Gel filtration through Sephadex G-75 revealed that wild-type and R91L regA proteins (14.6 kDa) elute at a mass of 29 kDa, consistent with the mass of a dimer. However, wild-type regA preincubated with gene 44RE (1:1) resulted in a complex that eluted at approximately 20 kDa, consistent with a regA monomer-RNA complex. Covalent crosslinking of surface lysines with glutaraldehyde confirmed that wild-type and R91L proteins exist as dimers and higher oligomers in solution. However, the addition of RNA to wild-type regA protein prior to crosslinking inhibited the formation of crosslinked dimers. Thus, the regA protein-protein interactions observed in solution are disrupted or blocked in the presence of gene 44RE RNA. Together, these studies demonstrate that regA protein binds RNA as a monomer, although free protein exists predominantly as a dimer.