Scientists decipher "fail-safe" system that limits gene copying in cells

tinct controls have to be turned off simultaneously for cells to start replicating again. This is unlikely to happen by accident, so this multi-layered protection is virtually fail-safe. That's what you want when there is no room for error."

While multiple overlapping pathways are not thought to be an uncommon safeguard, few such systems have been clearly described, Li said.

In yeast, as in human cells, DNA replication is triggered in the nucleus when proteins known as replication factors position themselves at hundreds to thousands of pre-determined sites along the DNA molecule. Li's UCSF team knew that a specific set of proteins called kinases played a central role in preventing this critical process from starting more than once. But they did not know just how the kinases acted - whether, for example, they triggered the replication proteins into action or helped block their action. Nor, of course, did they know how many steps were involved.

The scientists found that the kinases act on at least three replication factors, knocking them out of commission in different ways. A kinase causes one replication factor called Mcm to get expelled from the nucleus; another, known as Cd6, is chemically degraded. A third replication protein, called ORC for "origin recognition complex," is de-activated by kinase through a still-unknown process.

Kinases are ubiquitous enzymes widely used in cells to regulate other proteins. In a step known as phosphorylation, they add a phosphate molecule to their target, changing its shape or signaling other proteins to degrade, relocate or modify it.

Only when the scientists managed to block the effects of kinases on all three replication proteins -Mcm, Cd6 and ORV - were they able to trigger DNA replication to re-start in the cell. But even this did not lead to a complete doubling of the cell's genome. About 50 percent of the genes were re-copied, the researchers report, suggesting that still more regulat

Contact: Wallace Ravven
University of California - San Francisco

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