The research, by biochemistry professors Paul Doetsch and Daniel Reines, and graduate student Wei Zhou, is described in the Aug. 25, 1995, issue of the journal Cell.
Until now, many scientists have believed that only one of the two strands of DNA plays a key role throughout the process of cellular transcription, in which genetic information is copied from DNA to make RNA. The researchers discovered that in fact both, and not just one, of the two strands of DNA play key roles in the entire transcription process.
Cells are constantly performing transcription in order to manufacture proteins needed to maintain normal cellular functions. During transcription, the cell first makes an RNA copy of the DNA molecule by using an RNA polymerase - a specialized protein that reads the DNA genetic code imprinted on one of the two DNA strands. The polymerase then turns the genetic code into an RNA genetic code. The base sequence code on the RNA, in turn, serves as a blueprint that "codes" for a particular protein.
The scientists created pieces of DNA that were damaged in ways similar to a type of DNA damage created by radiation. Each damaged piece of DNA was missing progressively more bases - between one and 24 - leaving increasingly wide gaps in the strands.
They then conducted experiments in a test tube to see whether or not the gaps would interfere with the transcription process. They were surprised to find that the RNA polymerase was able to "jump" the gaps and continue reading the strand and complete the transcription process despite the damages. If the DNA was missing a certain number of bases, the newly made RNA had the same number of bases missing.
Since only one strand (called the template strand) of the double-stranded DNA is copied into RNA, most studies of the mechanics of gene expression have focused only on the template strand, and little attention was given to the other (non-template) strand.
The Emory scientists discovered that the non-template strand in their experiment actually was playing a heretofore hidden but crucial role in the transcription process. The polymerase they were using - bacteriophage T7 RNA polymerase - was able to bypass gaps in the template strand of RNA and continue the elongation process by binding to the non-template strand of DNA and using this strand to pull the template strand into the part of the polymerase that makes RNA. Without both DNA strands, the chain could not have continued to extend and transcription would have stopped.
Doetsch and his group are now testing transcription in live bacterial cells and mammalian cells to explore their new discovery further. If they find that this same process occurs in live cells, the new knowledge would pave the way for more studies delving into the mechanisms of normal gene expression as well as perhaps revealing a new pathway for producing proteins containing mutations.
"This new information has revealed a very important clue about the mechanisms of how all genes are potentially transcribed," said Doetsch.
-- Holly Korschun