New assay identifies southpaw DNA.

New assay identifies southpaw DNA 

Researchers have developed the first test capable of spotting oddly twisted, "left-handed' segments of DNA inside living cells. The technique may help settle some long-standing questions about the prevalence of such DNA inliving cells and the roles it may play in triggering disease.

Robert D. Wells and his colleagues at the University of Alabama at Birmingham developed the assay to identify segments of genetic material that sport a backwards twist. Their research, which appears in the Nov. 6 SCIENCE, may provide new insights into the molecular basis of such genetically influenced ailments as heart disease and cancer.

The new assay comes 34 years after James Watson and Francis Crick discovered the molecular structure of DNA. Since then, the double-stranded helix has become a sort of international logo, recognized by scientists and lay people alike as the three-dimensional blueprint of everything genetic.

Despite the apparent consensus regarding DNA's structure, however, some scientists have for years argued that some DNA has a left-handed rather than the classical right-handed twist (SN: 6/9/84, p.362). In fact, researchers have observed left-handed DNA in the test tube, thus verifying that such a conformation is possible. But no method existed to test for it in a living cell.

"There have been several thousand papers in the last eight years about test tube evidence of Z-DNA,' a particular variety of left-handed DNA, Wells says. However, he adds, it was important to see if it existed in living cells, because DNA that is twisted in reverse is "likely to be highly mutagenic.'

In particular, the regions where left-handed and right-handed twists conjoin may be "hot spots' for mutation, since DNA's twin strands might tend to unravel a bit in order to accommodate the change of direction. "That junction is a really weird structure,' Wells says.

In developing the assay, Wells relied upon previous test tube research that had shown how difficult it is to add a methyl group to left-handed segments of DNA. And without the addition of such a methyl group, he knew, certain enzymes would be unable to cleave the DNA into subunits.

With these rules in mind, the researchers incorporated into bacterial cells a variety of DNA sequences--some that had a known tendency to be left-handed, others that were right-handed. Later, they attempted to methylate, then cleave, the DNA in the cells, and looked for evidence of DNA subunits. A lack of subunits indicated that methylation-dependent cleavage had not occurred, demonstrating the presence of left-handed DNA.

The test, says Wells, is the first one to detect anything other than "the garden variety' of DNA in a living cell. "The next big thing will be to understand its function. Nobody thinks these unusual structures are inert in the cell.'

It is likely, he says, that such unusual DNA conformations may impair transcription of the genetic code. Repliation, recombination and cell repair may also be affected, and studies may someday show correlations between such unusual DNA structures and susceptibility to certain hereditary diseases.

Moreover, exposure to DNA-damaging forces, such as radiation, may either induce or exacerbate conformational changes that could then be inherited by offspring, he says. He notes that aflatoxin, a potent toxin found in some moldy foods, has been shown to prevent left-handed DNA from reverting to a right-handed form.