Two separate controls regulate chromosome copying in yeast

October 22, 2001

The crucial job of ensuring that just one copy of a genome gets made during cell division turns out to be shared by two independent "controllers," researchers from Johns Hopkins School of Medicine report in the Oct. 23 issue of the Proceedings of the National Academy of Sciences.

In experiments with yeast cells, the scientists discovered that if the two controller proteins remain in the cell then copying continues abnormally. Normally, the proteins are destroyed after a single copy, or replication, of the DNA is made.

"We knew these proteins were required for DNA replication and that they normally went away after one DNA copy was made, but we didn't know whether their disappearance was important for controlling the duplication of the genome," says Thomas Kelly, M.D., Ph.D., professor and director of molecular biology and genetics and director of the Institute for Basic Biomedical Sciences. "Now we know that DNA replication ceases because these two proteins are destroyed."

Understanding this complex and tightly regulated process may help clarify what goes wrong in cancer cells, say the researchers. The yeast used in the experiment (Schizosaccharomyces pombe) divides by splitting into two new cells, each with a copy of the organism's entire genome, in a process very similar to that of human cells.

"In human cells, for reasons still largely unknown, some cells ultimately acquire enough genetic mutations to cause cancer, despite having different ways to prevent, find and fix problems in the genome," says Mark Frattini, M.D., Ph.D., a postdoctoral fellow in the department of medical oncology and the department of molecular biology and genetics, which is part of the school's Institute for Basic Biomedical Sciences. "We can't yet link these two controller proteins in yeast or their human counterparts to cancer, but we do have a new genome control pathway to examine."

In the yeast, one controller, the Cdc18 protein, helps build machinery that copies DNA, and the other, Cdt1, helps start that machine. The levels of both proteins normally rise before DNA replication and fall once it's completed.

By disrupting the yeasts' ability to regulate the levels of Cdc18 and Cdt1, the Hopkins scientists proved that the normal destruction of the two proteins restricts DNA replication to a single copy. In cells with mutant Cdc18 and Cdt1 whose levels never drop, DNA replication keeps going.

"Having two proteins offers redundant protection against making extra copies of DNA, which helps maintain the integrity of the genome for subsequent generations of cells," says Frattini.

The scientists used a version of the gene for Cdc18 that would produce normal amounts of an altered protein that could not be marked for destruction, but which would otherwise function normally. In the case of Cdt1, the scientists created two new versions of the gene, both of which were controlled by an "on switch" that the researchers, but not the cell, could manipulate.

The mutations worked as expected: the levels of Cdc18 and the Cdt1 proteins no longer varied normally. Cdc18 levels increased when the cell began constructing the DNA copier, as normal, but then remained high. And all cells produced Cdt1, regardless of their point in cell division.

The amount of DNA in the cells reflected the changes' effects on the DNA copier. Many cells with the mutant Cdc18 and Cdt1 contained as much as two to four times the amount of DNA they should have, says Frattini. Cells with just one of the proteins altered were normal.

Other researchers have found that very large amounts of indestructible Cdc18 could lead to extra replication of DNA in the yeast, but Hopkins' experiments show that if protein levels are closer to normal, both Cdc18 and Cdt1 are required to continue DNA copying.
In addition to Kelly and Frattini, co-authors are first author Vidya Gopalakrishnan, Pamela Simancek and Christopher Houchens of The Johns Hopkins University School of Medicine, Hilary Snaith formerly of The Salk Institute and now at the University of Edinburgh, UK, and Shelley Sazer of the Baylor College of Medicine.

Related Web sites:

Media Contact: Joanna Downer 410-614-5105 Email:

Johns Hopkins Medical Institutions' news releases are available on an EMBARGOED basis on EurekAlert at, and from the Office of Communications and Public Affairs' direct e-mail news release service. To enroll, call 410-955-4288 or send e-mail to

On a POST-EMBARGOED basis find them at

Johns Hopkins Medicine

Related Cancer Articles from Brightsurf:

New blood cancer treatment works by selectively interfering with cancer cell signalling
University of Alberta scientists have identified the mechanism of action behind a new type of precision cancer drug for blood cancers that is set for human trials, according to research published in Nature Communications.

UCI researchers uncover cancer cell vulnerabilities; may lead to better cancer therapies
A new University of California, Irvine-led study reveals a protein responsible for genetic changes resulting in a variety of cancers, may also be the key to more effective, targeted cancer therapy.

Breast cancer treatment costs highest among young women with metastic cancer
In a fight for their lives, young women, age 18-44, spend double the amount of older women to survive metastatic breast cancer, according to a large statewide study by the University of North Carolina at Chapel Hill.

Cancer mortality continues steady decline, driven by progress against lung cancer
The cancer death rate declined by 29% from 1991 to 2017, including a 2.2% drop from 2016 to 2017, the largest single-year drop in cancer mortality ever reported.

Stress in cervical cancer patients associated with higher risk of cancer-specific mortality
Psychological stress was associated with a higher risk of cancer-specific mortality in women diagnosed with cervical cancer.

Cancer-sniffing dogs 97% accurate in identifying lung cancer, according to study in JAOA
The next step will be to further fractionate the samples based on chemical and physical properties, presenting them back to the dogs until the specific biomarkers for each cancer are identified.

Moffitt Cancer Center researchers identify one way T cell function may fail in cancer
Moffitt Cancer Center researchers have discovered a mechanism by which one type of immune cell, CD8+ T cells, can become dysfunctional, impeding its ability to seek and kill cancer cells.

More cancer survivors, fewer cancer specialists point to challenge in meeting care needs
An aging population, a growing number of cancer survivors, and a projected shortage of cancer care providers will result in a challenge in delivering the care for cancer survivors in the United States if systemic changes are not made.

New cancer vaccine platform a potential tool for efficacious targeted cancer therapy
Researchers at the University of Helsinki have discovered a solution in the form of a cancer vaccine platform for improving the efficacy of oncolytic viruses used in cancer treatment.

American Cancer Society outlines blueprint for cancer control in the 21st century
The American Cancer Society is outlining its vision for cancer control in the decades ahead in a series of articles that forms the basis of a national cancer control plan.

Read More: Cancer News and Cancer Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to