Conditional rifampicin sensitivity of a rif mutant of Escherichia coli: rifampicin induced changes in transcription specificity.

ABSTRACT

A rif mutant of Escherichia coli that exhibits medium and temperature-dependent sensitivity to rifampicin is described. In the absence of rifampicin, this strain grows in minimal and rich media at 30°C and 42°C. In its presence it is viable in rich medium at both temperatures, but in minimal medium only at 30°C. In minimal-rifampicin medium at the higher temperature, RNA synthesis is decreased. The addition of certain divalent salts (MgSO4, CaCl2, BaCl2) in excess, or chelators (EDTA, EGTA, o-phenanthrolein) greatly increase viability in minimal-rifampicin medium at 42°C. Excess MgSO4 (10 mM) also increases the rate of RNA synthesis in the same medium. A model is proposed wherein the rif mutation is suggested to cause a structural change in RNA polymerase that allows the binding of rifampicin and other ligands at 42°C. Rifampicin-binding is suggested to alter the conformation of RNA polymerase, impairing its ability to express genes required for growth in minimal medium. Implicit in this view is the assumption that these genes are structurally different from those expressed in rich medium in respect of certain template features recognized by RNA polymerase.