Scientists have shown that cells' DNA-reading machinery can skim through certain kinds of damaged DNA without skipping any letters in the genetic "text." The studies, performed in bacteria, suggest a new mechanism that can allow bacteria to develop resistance to antibiotics.
The results were published online this week in the Proceedings of the National Academy of Sciences. The senior author is Paul Doetsch, PhD, professor of biochemistry and radiation oncology at Emory University School of Medicine and associate director for basic research at Winship Cancer Institute of Emory University.
Working with Doetsch, graduate student Cheryl Clauson examined the ability of RNA polymerase (the enzyme that transcribes, or makes RNA from DNA) to handle damaged DNA templates.
RNA polymerase reads one strand of the double helix and assembles RNA that is complementary to that strand. In test tube experiments, when the enzyme comes to a gap or a blank space, it keeps reading but leaves out letters across from the damaged stretch. In contrast, in cells, RNA polymerase puts a random letter (preferring A) across from the gap.
"We were surprised to find that the transcription machinery rolls right over the damaged portion," Doetsch says. "This shows that if the cell initiates, but doesn't complete repair, it still can lead to mutagenesis."
Clauson says a challenge in planning her experiments was finding a way to sensitively detect when RNA polymerase reads through DNA damage.
She loaded damaged DNA into a gene that encodes an enzyme from fireflies, which generates light-emitting chemicals, and then introduced that gene into bacteria. A full working enzyme is produced only if RNA polymerase bypasses the DNA damage without skipping any letters.
DNA in every type of cell, whether bacterial, plant or animal, is constantly being damaged by heat, oxygen and radiation. In addition, all cells make RNA from some of their
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