The researchers found a distinct day/night cycle: The plasmid is smaller and more tightly wound during periods of light than they are during periods of darkness. They also found that this rhythmic condensation disappears when the cyanobacteria are kept in constant darkness.
This is one of the first pieces of evidence that the biological clock exerts its effect on DNA structure through the coiling of the chromosome and that this, in turn, allows it to regulate all the genes in the organism, says Woelfle.
Some cyanobacteria use their biological clocks to control two basic processes. During the day, they use photosynthesis to turn sunlight into chemical energy. During the night, they remove nitrogen from the atmosphere and incorporate it into a chemical compound that they can use to make proteins.
According to the Johnson labs oscilloid model, the genes that are involved in photosynthesis should be located in regions of the chromosome that are turned on by the tighter coiling in the DNA during the day and turned off during the night when the DNA is more relaxed. By the same token, the genes that are involved in nitrogen fixation should be located in regions of the chromosome that are turned off during the day when the DNA is tightly coiled and turned on during the night when it is more relaxed.
The researchers see no reason why the bioclocks in higher organisms, including humans, do not operate in a similar fashion. This could be a universal theme that we are just starting to decipher, says Woelfle.
The DNA in higher organisms is much larger than that in cyanobacteria and it is linear, not circular. Stretched end-to-end, the genome in a mammalian cell is about six feet long
|Contact: David F. Salisbury|