A strange new genetic entity has been discovered in methane-eating microbes, and it could help fill in a gap in our understanding of Earth鈥檚 climate.
Named 鈥淏orgs鈥� after Star Trek aliens that assimilate the biology of other creatures, these enigmatic stretches of DNA inhabit single-celled organisms called archaea, where they appear to acquire and swap genes and potentially boost their hosts鈥� ability to consume methane, one of the most potent greenhouse gases.
鈥淭hese Borgs seemingly represent a new type of genetic element,鈥� says at Wageningen University & Research in the Netherlands, who wasn鈥檛 involved in the study.
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快猫短视频s have long known about聽mobile genetic elements, pieces of genetic material that can either move around a host鈥檚 genome or sit alongside it and travel between cells. They include plasmids 鈥� circular bits of DNA that microbes often exchange with each other 鈥� and phages, viruses that infect bacteria and archaea.
at the University of California, Berkeley, and her colleagues were using DNA sequencing to study microbial communities living in oxygen-free mud when they stumbled across some unusual stretches of DNA. The team painstakingly stitched these together and reconstructed the full sequences of four Borg elements, naming them Purple, Black, Sky and Lilac. Studies of other samples revealed the existence of a further 15 types.
鈥淭he thing that strikes you first is, they鈥檙e huge. I mean really huge,鈥� says Banfield. At up to about 1 million DNA letters long 鈥� a third the size of their host鈥檚 genome 鈥� they are the largest mobile genetic element yet found in archaea or bacteria.
Their structure is strange too: for example, unlike circular plasmids, Borg DNA is linear and contains many sections of repeated sequences. None of the proteins coded in the Borg genomes look like proteins seen in plasmids or viruses, nor do the DNA replications start in the usual sites seen in known plasmids. All in all, they look like something completely new. Some Borgs use CRISPR as a kind of microbial immune system, possibly aimed at zapping rival mobile genetic elements in the host. Similar features individually crop up in other genomes, but never before together like this.
The large number of protein-coding genes that Borgs contain is also remarkable. While the majority of these are unknown, about a fifth resemble genes found in archaea 鈥� particularly those of a methane-consuming genus called Methanoperedens in which the Borgs replicate. As well as assimilating host genes, there are signs that different kinds of Borg can swap DNA with each other and possibly move between hosts, potentially allowing Methanoperedens to acquire new genes from elsewhere.
Borgs, particularly the Lilac Borg, contain genes that could turbocharge the Methanoperedens鈥檚 ability to consume methane. This is intriguing because the balance between methane production and microbial methane consumption directly affects climate, says at Case Western Reserve University in Cleveland, Ohio. Consequently, scientists want to understand what affects the amount of methane these microbes can consume and how that may change over time.
Borgs may be a missing piece in this puzzle, but right now there is still a lot to learn. Banfield is on the case, planning experiments to explore the origin and function of Borg proteins. 鈥淭here could be genome-editing tools long term, but it鈥檚 really too early to speculate exactly what they might be at this time,鈥� she says. 鈥淏ut no doubt there is a huge potential for new information, new biochemistry.鈥�
Reference: Biorxiv, DOI:
