Structure–Function Relationships inEscherichia coliTranscription Termination Protein Rho Revealed by Radiation Target Analysis

High-energy electrons were used to measure the target sizes for inactivation of the RNA-dependent ATPase activity ofEscherichia colitranscription termination factor Rho, for its ATP binding ability, and for its physical destruction. SDS–PAGE analysis of irradiated samples indicated that the target size for polypeptide destruction in the homohexameric enzyme is the dimer, indicating that energy transfer must occur from a hit subunit to one other subunit, although the subunits are not known to be linked by any covalent bonds. The ATP binding ability of Rho also inactivates as a dimer, a result that is consistent with the physical destruction target size. However, a single subunit as the ATP binding entity is not excluded. The RNA-dependent ATPase activity of Rho inactivates with the apparent target size of trimer to tetramer, indicating that interactions among the subunits of Rho are required for ATP hydrolysis. Rho hexamers are known to exchange subunits, although the identity of the exchanging unit is not known. Models in which this property of Rho is taken into account indicate that the closest fit to the experimental data is for an ATPase target size of a hexamer with dimers as the exchanging units, consistent with earlier chemical inactivation studies.