The interaction of RNA polymerase and DNA. Effects on the helix-coil transition and light scattering

To characterize the interactions of RNA polymerase with DNA, we have investigated the thermal transition of poly[d(A-T] bound to RNA polymerase from Escherichia coli and the aggregation properties of the enzyme with DNA. The melting curve of the DNA-enzyme complex demonstrates a sharply lowered melting temperature for part of the DNA, whereas for another fraction the double helix is stabilized. This indicates that the DNA-binding site of RNA polymerase serves two functions: (1) to disrupt the double helix at one point, and (2) to maintain the duplex form at other points. The aggregation of DNA and RNA polymerase has been monitored by turbidity measurements, and conditions have been delineated under which aggregation is minimized. Holoenzyme added to double-stranded DNA or single-stranded DNA has little or no tendency to aggregate under most conditions. Core enzyme, on the other hand, aggregate extensively with double-stranded DNA, the only under conditions of low salt (10 mM KCl), without Mg2+, or at high salt (300 mM KCl), with or without Mg2+, can this aggregation be eliminated. Core enzyme also does not aggregate in the presence of single-stranded DNA. These aggregation properties are interpreted as evidence for more than one DNA-binding site on RNA polymerase.