Electron microscopic studies of the binding of Escherichia coli RNA polymerase to DNA. I. Characterization of the non-specific interactions of holoenzyme with a restriction fragment of bacteriophage T7 DNA

Non-specific interactions between a 3800 base-pair restriction fragment of bacteriophage T7 DNA (MboI-C) and Escherichia coli RNA polymerase holoenzyme have been examined by electron microscopy. Holoenzyme displays a relatively weak and rapidly reversible binding to DNA that is only slightly reduced at elevated salt concentrations. As the concentration of NaCl is increased from 50 mm to 200 mm, the binding constant decreases from 2 × 10⁴m^?1to 4 × 10³m^?1. It is concluded that only 1 to 2 sodium ions are released from the DNA when holoenzyme binds non-specifically.The validity of the electron microscopic technique for determining binding constants has been investigated by varying aspects of the grid surface and by examining the non-specific interactions of lac repressor with DNA.