Bacteria host bits of DNA that can replicate and spread to other bacteria. Now, researchers have discovered that these 鈥渟elfish genes鈥� wage war on their rivals using DNA-destroying CRISPR systems.
CRISPR has become famous as a way of editing genes, but in bacteria, CRISPR systems often act as an immune system, targeting and dismantling the DNA of invading viruses.
at the University of Copenhagen in Denmark and his colleagues have shown that some of the self-replicating bits of DNA in bacteria encode their own CRISPR systems, and that most of these target other, similar bits of DNA.
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The main genome of simple cells such as bacteria usually consists of a large circle of DNA containing thousands of genes. In addition, many bacteria harbour smaller circles of DNA called plasmids, which may contain only a few dozen genes.
Plasmids can replicate separately from the main genome, so there can be many copies in a single cell. They are one type of what are known as mobile genetic elements, and they actively spread to other bacteria 鈥� including to other species.
鈥淭hey are independent or semi-independent genetic entities,鈥� says Pinilla-Redondo.
Plasmids often carry genes for traits that benefit their host cells, and thus themselves, such as antibiotic resistance. But they can also be parasitic, exploiting a host cell鈥檚 resources for their own benefit.
Some plasmids also contain the CRISPR systems encoded in the main genomes of bacteria. Pinilla-Redondo and his colleagues have now done the first comprehensive study of the CRISPR systems encoded by plasmids, looking at more than 17,000 plasmid sequences available in databases.
The team found that 3 per cent of these plasmids encode CRISPR systems, a relatively large proportion. But the big surprise was that rather than protecting against viruses, most of these CRISPR systems targeted other plasmids.
鈥淲e thought they would help the hosts survive viral infections,鈥� says Pinilla-Redondo. 鈥淏ut it looks like primarily their focus is competition with other plasmids.鈥�
So, some plasmids seem to use CRISPR to destroy other plasmids that enter their host cells, to prevent them competing for resources. Alternatively, a plasmid entering a cell might use its CRISPR system to chew up other plasmids already present.
at the US National Center for Biotechnology Information in Maryland, thinks this is the right conclusion. 鈥淚 wholeheartedly agree,鈥� he says.
Koonin has previously suggested that defence systems such as CRISPR that can be acquired by mobile genetic elements such as plasmids and co-opted for their own purposes.
This traffic goes both ways, with host cells sometimes acquiring these systems back from plasmids.
This is part of a paradigm shift in the field, says Pinilla-Redondo. It used to be assumed that defence systems against mobile genetic elements were there for the benefit of the host cells, he says.
But work like this shows that a lot of these defence systems are there for the benefit of other mobile genetic elements instead. 鈥淭here鈥檚 a lot of warfare going on between the parasites,鈥� says Pinilla-Redondo.
Reference: bioRxiv,
