When the going gets tough, stress response genes can make do with less
La Jolla, CA – Reading the DNA recipes of the 25,000 or so genes within a human cell, a process called transcription, is a highly scripted endeavor. Like the main character in a movie, an enzyme called RNA Polymerase plays the lead role and is supported by an ever-changing cast of supporting transcription factors that come and go as the script demands.
But, in the case of an emergency, cells swiftly dispense with the regular script and rely on a minimalist bypass strategy to 'read' stress response genes that help cells deal with the stressful situation, scientists at the Salk Institute for Biological Studies have discovered.
"When things start to go wrong, there's no time to think," said Beverly Emerson, Ph.D., professor in the Salk Institute's Laboratory or Regulatory Biology and one of the lead authors of the research report. "RNA Polymerase has to be ready to go at the slightest provocation," she said.
Their findings, published in this week's issue of the journal Genes and Development, may also explain how the latest generation of chemotherapeutic drugs exert their deadly effects.
"Even when we treat cells with a drug that shuts down general transcription, some stress response genes are still transcribed," said former Salk post-doctoral researcher Joaquin Espinosa, Ph.D., now an assistant professor at the University of Colorado, Boulder. "Under these extreme circumstances the cellular transcription machinery dispenses with many factors that were thought to be absolutely essential," said Espinosa.
As a first-responder, the transcription factor p53 is called to action when cells experience stressful conditions such as DNA damage. Depending on the situation, p53 then turns on genes that halt cell division to allow time for repairs or, when all rescue attempts prove futile, order the cell to commit suicide.
But no matter what goes wrong in the cell, stress response genes need to be transcribed. So, p53 has to rely on unorthodox methods to get its job done. "p53 is like a firefighter that rushes into a burning building, when everybody else tries to leave. But they are prepared because they have specialized equipment such as a special key that allows them to operate the elevator when nobody else can," said Espinosa. "And p53 can initiate the transcription of certain genes when everything else shuts down," he said.
In an earlier study, the Salk team had discovered that RNA Polymerase and p53 are already waiting at the starting blocks of genes that have to respond quickly in cases of cellular emergencies. P21CIP1 is one of those first-responder genes that are preloaded with RNA Polymerase and other components of the transcription machinery, waiting for the start signal.
Under normal circumstances, before it can start transcribing a gene, RNA Polymerase needs to be modified by an enzyme that attaches groups of molecules called phosphates. En route, it has to recruit several so-called elongation factors to finish the job. Paradoxically, when the Salk scientists shut down general transcription in colon cancer cells by using the drug DRB (5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole), the enzymatic modification of RNA Polymerase and the help of certain elongation factors was no longer necessary for p21CIP1 to be read and to initiate the cellular suicide program.
"We had no idea that such a bypass system existed because nobody expected that a gene could be transcribed in the absence of known elongation factors and RNA Polymerase phosphorylation," said Emerson. "But now, we also have to look for mutations in the bypass system because they may explain why some cancer cells don't commit suicide when they are supposed to," she added.
Drugs that shut down general transcription are currently under development for the treatment of cancer. The newly discovered bypass system explains how this new generation of drugs force cancer cells to commit suicide.
Scientists who also contributed to this study include first author Nathan Gomes, Glen Bjerke and Stephanie Szostek at the University of Colorado, Boulder, and Briardo Llorente at the Salk Institute.
The Salk Institute for Biological Studies in La Jolla, California, is an independent nonprofit organization dedicated to fundamental discoveries in the life sciences, the improvement of human health and the training of future generations of researchers. Jonas Salk, M.D., whose polio vaccine all but eradicated the crippling disease poliomyelitis in 1955, opened the Institute in 1965 with a gift of land from the City of San Diego and the financial support of the March of Dimes.