Enzyme takes the DNA lead

Researchers have reported an important discovery about a critical new role that the enzyme DNA polymerase epsilon plays in replicating DNA in higher organisms such as yeast and perhaps even humans.

The researchers from the US National Institute of Environmental Health Sciences (NIEHS) and Umea University in Sweden used an innovative strategy to demonstrate that in bakers yeast, DNA polymerase epsilon has a primary role in replicating the leading strand of DNA.

DNA polymerase epsilon was found to be a key determinant of genome stability and of cellular responses to DNA damage resulting from exposures to environmental stress.

The researchers built on fundamental discoveries on the structure and replication of DNA made by Nobel laureates James Watson, Francis Crick and Arthur Kornberg.

When Watson and Crick first described the structure of DNA in 1953, they pointed out that the two DNA strands, which are referred to as leading and lagging, pair with each other to form the now familiar double helix.

Shortly thereafter, Kornberg and colleagues discovered the first enzymes capable of replicating DNA, a process required to make new genomes for cell division. These enzymes, called DNA polymerases, were shown to copy the two DNA strands in only one of two possible directions.

One strand of the double helix must be replicated first by a dedicated leading strand polymerase, followed slightly thereafter by replication of the lagging strand by a different polymerase.

In lower organisms like the E. coli bacteria that Kornberg studied, one DNA polymerase can accomplish both tasks. However, humans and related higher organisms, such as bakers yeast, are much more complicated.

Recent discoveries, several of which emerged from the human genome project, indicate that the human genome encodes at least 15 DNA polymerases that can copy DNA.

Several of these are thought to perform genomic replication, while others operate under special circumstances, such as the repair of DNA damage resulting from environmental exposures.

"Amazingly, more than a half century after Watson and Crick first described the DNA double helix, it had remained unclear which of these many DNA polymerases in higher organisms is actually responsible for first replicating the leading strand during nuclear genome duplication," Dr Thomas Kunkel, one of the paper's authors and chief of the Laboratory of Structural Biology at NIEHS, said.

Kunkel said that the general strategy used in the study can now be applied to investigate other reactions that are critical for genome stability, including the identity of the lagging strand polymerase and the roles of more specialized DNA polymerases in copying damaged DNA.

According to first author Dr Zachary Pursell, a researcher in the DNA Replication Fidelity Group at NIEHS, the study's findings advance the fundamental understanding of how the genomes of many higher organisms are replicated.

The study was published in this week's issue of Science.

Source: NIH/National Institute of Environmental Health Sciences