( ese replication stop signs and keep c...)Little Known DNA Repair Enzyme May Be A Tumor Suppressor Gene,Health

ese replication "stop signs" and keep cells dividing that would otherwise be killed off by their own suicide mechanisms.

First discovered in budding yeast cells, and later in plants and animals, pol zeta has the remarkable ability to efficiently extend, in the test tube, DNA with lesions that stop most other DNA polymerases in their tracks. Research has shown that inactivation of this lesion-replicating enzyme in yeast leads to a dramatic decrease in the frequency of mutations induced by a wide range of DNA damaging agents.

In this study, Dr. Wittschieben and his associates sought to determine pol zeta's key role in mice cells. To do this, they disabled or "knocked out" the gene for pol zeta's Rev3L subunit, the part with the lesion-replicating capabilities. However, knocking out the Rev3L gene proved lethal to the mice embryos. All the same, fibroblasts were isolated from these embryos to see if they could be kept alive in culture. After repeated attempts, the mouse embryonic fibroblasts, or MEFs, failed to divide and died within a few weeks or months.

Suspecting that the MEFs died because they were self-destructing, or undergoing apoptosis, the investigators then knocked out the gene for a protein known as p53, which is a cell-suicide-signaling molecule. After matings between the p53 knockout mice and Rev3L knockout mice, the investigators isolated and cultured MEFs from all the offspring of the matings to see if any would grow. To start with, the cells all failed to divide. However, three months later, some cells began to grow and at a surprisingly robust rate.

Dr. Wittschieben said, "Since the only Rev3L-deficient cells that began dividing also were p53 deficient, we believe that knocking out their apoptotic mechanism was key to this viability. However, they didn't begin dividing right away, so something else must have happened. We are still not sure what it is."

When the investigators wondered why these cell
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