Evidence for an interwound form for the superhelix in circular DNA

✍ Scribed by George W. Brady; David Foos; C. J. Benham

• Book ID: 101720674
• Publisher: Wiley (John Wiley & Sons)
• Year: 1984
• Tongue: English
• Weight: 196 KB
• Volume: 23
• Category: Article
• ISSN: 0006-3525
• DOI: 10.1002/bip.360231219

✦ Synopsis

In a previous communication' we reported the results of a small-angle xray study of dissolved superhelical DNA. Among other findings, the results indicated that the superhelix existed in the noninterwound form. Since the x-ray intensity pattern is the Fourier transform of the electron-pair distribution, the technique is a very direct method for determining the conformation right in solution. The other "direct" method is electron microscopy (EM), but this method requires that the sample be spread flat and dried, so that the structure observed may not be truly representative of the actual molecule in solution. This limitation being stated, the EM results seem to favor the interwound form for the helix? Thus, results from the two methods disagree, and this is a cause for concern. If, and until, we can establish that the EM sample preparation is the culprit, any observation that could shed light on the disagreement is useful. In this Research Communication we report on some new experiments indicating that in some few case (5%) the x-ray patterns also show the presence of the interwound form for the helix. We have not yet been able to pinpoint the reasons for this anomalous result, but we feel it is important enough to report now. The fact that the two forms have been observed using the same direct x-ray technique is compelling evidence that the existence of two forms for the superhelix in solution is not an artefact.

* To whom reprint requests should be addressed at the Wadsworth Center for Laboratories and Research.