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Tuesday, December 10, 2013

Bacteria add ancient DNA to their genomes

When an organism dies, the DNA within its cells is released to the environment. Once there, the long DNA strands are quickly broken apart and degraded until almost all the fragments are shorter than 100 nucleotides. These DNA fragments are a source of food for bacteria, but apparently that’s not all the microbes can do with them. According to Søren Overballe-Petersen from the University of Copenhagen and his colleagues, these bits of DNA could be driving bacterial evolution.

The researchers found that the soil bacteria Acinetobacter baylyi weren’t just consuming small bits of DNA. The microbes were integrating the fragments into their own genomes via a process called 'transformation'. This was true even if the DNA was damaged with nicks or gaps, and even if it was as small as 20 base pairs long. 

Illustration of bacterial transformation. DNA from dead cells gets cut into fragments and exits the cell. The free floating DNA can then be picked up by competent cells.

Especially intriguing, the bacteria were able to incorporate DNA recovered from a 43,000 year old woolly mammoth. You might think that ancient DNA rarely becomes available to modern bacteria, but tons of the stuff is constantly being released from river sediments.

Not all types of bacteria can take up DNA in this manner. But for those that can, virtually any sequence of DNA on Earth is available for inclusion in that cell’s genome. After all, if you’re only talking about small snippets of DNA, chances are some animal has died and released that fragment within the past few thousand years.

Just to be clear, these small slivers of DNA are much too small to add new genes to the bacterial repertoire. Your average gene is at least ten times longer than the largest of these fragments. They can however, alter the bacterial genome, much like any other type of mutation would do. Thus, the incorporation of small bits of DNA, ancient or otherwise, may be yet another way that bacteria can evolve so rapidly.

Søren Overballe-Petersena, Klaus Harms, Ludovic A. A. Orlando, et al (2013). Bacterial natural transformation by highly fragmented and damaged DNA Proceedings of the National Academy of Sciences of the United States of America DOI: 10.1073/pnas.1315278110.

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