Conformational changes in deoxyribonucleic acid during transcription

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Abstract
Circular dichroism (CD) was used to examine changes in secondary structure of calf thymus DNA during in vitro transcription. Formation of a binary complex between DNA and RNA polymerase (nucleoside triphosphate:nucleotidyltransferase, EC 2.7.7.6) did not alter the CD spectrum of the DNA. Alterations in ellipticity in the spectral region between 245-300nm occurred during synthesis of RNA. This change was consistent with a B- to A-like form transition in polynucleotide conformation. The increment of ellipticity consisted of 2 separate components; component 1 was insensitive to treatment with pancreatic RNase whereas component 2 was a ribonuclease labile fraction. Cleavage by restriction endonucleases did not produce or significantly alter the ellipticity of transcription. Between 50-60% of the component 1 ellipticity was sensitive to pancreatic DNase. Apparently, component 1 is a property of DNA and an alteration in secondary conformation which affects this component extends cooperatively beyond the DNase 1 insensitive DNA-RNA polymerase complexes.