Physicochemical studies on interactions between DNA and RNA polymerase. Ultraviolet absorption measurements

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Abstract
The interaction between Escherichia coli RNA polymerase and a restriction fragment of coliphage T7 DNA containing four promoter sites for the coli enzyme has been studied by difference uv absorption spectroscopy in a low ionic strength buffer containing 10 mM MgCl 2 and 50 mM KCl. The binding of the en zyme to the DNA is accompanied by a hyperchromic shift which shows a maximum around 260 nm, and increases with increasing temperature in the temperature range studied (4–40°C). Measurements were also carried out with whole T7 DNA and a restriction fragment containing no promoter site. A comparison of the results obtained with the various DNAs suggests that the binding of an RNA polymerase to a promoter site in the low ionic strength medium causes the disruption of a short segment of the DNA helix, of the order of ten pairs; the binding of an enzyme molecule to a promoter site appears to have a cooper ative effect on the binding of enzyme molecules to adjacent non-promoter sites with concomitant disruption of DNA base pairs.