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Thermal unwinding of simian virus 40 transcription complex DNA.

Two long-standing questions in the control of eukaryotic gene expression have been how the structure of transcribing chromatin compares with that of nontranscribing chromatin and how chromatin structure differs among various eukaryotic organisms. We have addressed aspects of these two questions by characterizing the rotational flexibility of the DNA of the simian virus 40 (SV40) transcription complex. When transcription complex samples are incubated with topoisomerase at 0 degrees C or 37 degrees C, the DNA of the 37 degrees C sample is unwound by 1.8 turns relative to that of the 0 degrees C sample. This amount of unwinding is similar to that observed for bulk, untranscribed SV40 minichromosome DNA, indicating that the chromatin structure of a transcribed gene resembles that of a nontranscribed gene in the degree of constraint that it imposes on its DNA. However, this amount of unwinding differs substantially from the value observed for yeast plasmid chromatin DNA, suggesting that yeast chromatin differs significantly from mammalian chromatin in this fundamental property.