Heat-loving Microbe Repairs Its DNA With Light

May 19, 1996

A University of Cincinnati biologist has found a microbe which not only survives under extremely hot and acidic conditions; it can repair its damaged DNA using natural light.

Sulfolobus acidocaldarius is one of a special group of micro-organisms, commonly called "hyperthermophiles." These organisms require extremely high temperatures to grow and reproduce, and they have been recovered by microbiologists over the past 25 years from various hot springs, undersea vents, and other geothermal environments.

When its genetic material (DNA) is artificially damaged by ultraviolet (UV) light, S. acidocaldarius has been found to repair the damage. However, Dennis Grogan, assistant professor of biological sciences at the University of Cincinnati, found that the repair process requires assistance from the environment, in the form of visible light. This light-driven repair of DNA, known to microbial geneticists as "photoreactivation," is the first DNA repair process to be measured in a hyperthermophile.

Grogan presented his findings May 19, 1996 at the 96th General Meeting of the American Society for Microbiology. He has been investigating the basic cellular properties of S. acidocaldarius for the past six years.

In nature, S. acidocaldarius thrives in acidic, sulfurous hot springs; in the laboratory, it prefers acidic growth medium (pH=3) at about 80 degrees Celsius. Such severe conditions often cause DNA and other cellular materials to decompose. So, Grogan has been looking at how this microbe managed to protect itself.

"Since the precise structure of DNA is essential for life and reproduction, it would seem logical that the S. acidocaldarius cell should have particularly effective ways of repairing damage to its DNA," said Grogan.

Grogan tested the microbe's repair capabilities by artificially damaging the DNA of live S. acidocaldarius cells with UV radiation, whose effects on DNA are precisely known. As expected, the vast majority of cells were unable to reproduce following this treatment.

If the cells were then exposed to visible light, however, a significant fraction of the damaged cells revived. Interestingly, the DNA repair process leading to this recovery works well at room temperature, which is far below the temperatures at which metabolic processes of hyperthermophiles normally occur.

"Photoreactivation has been observed in a variety of micro-organisms," said Grogan, "but its discovery in S. acidocaldarius raises particular questions about what it takes to survive in geothermal habitats. For example, are we to infer that UV radiation is a significant threat to survival in these high- temperature environments?"

On one hand, it would seem logical that S. acidocaldarius, like many organisms, should use the energy of the abundant, visible portion of the sun's spectrum to reverse the harmful effects of the UV that sunlight also contains. On the other hand, Grogan says it seems ironic that the first example of DNA repair to be observed in S. acidocaldarius works on UV-induced DNA damage, and not the damage expected to be caused by extremely high temperatures.

Grogan's research is supported with funding from the U.S. Office of Naval Research. Plans for future work on this topic include determining which wavelengths of light are effective and exploring ways in which this phenomenon can be used in the genetic analysis of DNA repair and mutation at extremely high temperatures.

University of Cincinnati

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