Rho transcription factor: symmetry and binding of bicyclomycin

The antibiotic bicyclomycin inhibits rho-dependent termination processes by interfering with RNA translocation by preventing RNA binding at the translocation site or by uncoupling the translocation process from ATP hydrolysis. Previous studies have shown that bicyclomycin binds near the ATP hydrolysis pocket on rho. The hexameric structure of rho indicates that it is in a class of enzymes with strong sequence similarity to F(1)-ATP synthase. The bicyclomycin derivative 5a-formylbicyclomycin, an inhibitor comparable to bicyclomycin, was previously shown to form a stable imine with rho and when reduced to the amine with NaBH(4) to singly label five of the six rho subunits. Lysine-336 was identified by mass spectrometric analysis of trypsin-digested fragments as the site of 5a-formylbicyclomycin adduction. A model of rho was made by threading the rho sequence on the known crystal structure of the alpha and beta subunits of F(1)-ATP synthase. The model, along with information concerning the extent and site of 5a-formylbicyclomycin adduction, indicates an overall C6 symmetry for rho subunit organization. We propose that the sequence similarity between rho and F(1)-ATP synthase extends to a similar quaternary structure and an equivalent enzyme mechanism. The proposed mechanism of RNA translocation coupled with ATP hydrolysis changes the overall symmetry of rho from C6 to C6/C3.