Borg (microbiology)


Borgs are large extrachromosomal linear DNA elements found in methanotrophic archaea that live in oxygen-starved environments such as deep mud. They have been found in organisms isolated from wetland, aquifer, and riverbed environments, as well as a deserted mercury mine, in California and Colorado. They were first described by Basem Al-Shayeb and Jill Banfield in 2022.
The nature of Borgs remains unclear; they are thought to be "giant linear plasmids" or giant viruses. At least 19 different types have been identified, all of which co-occur within Methanoperedens, which shares many of their genes. Methanoperedens' main chromosome is only about three times larger than the Borgs it hosts. It is speculated that Borgs may augment Methanoperedens' capacity for anaerobic oxidation of methane and protein production.

Discovery

Borgs were discovered on March 8, 2020 by Jill Banfield and her research group at the University of California, Berkeley. The name "Borg" was chosen as a reference to the Star Trek faction of the same name, due to the novel genetic elements' apparent propensity to assimilate genes from microbes, most notably Methanoperedens; the name was originally suggested by Banfield's son.

Features

The structures of Borg genomes are conserved and distinct from the plasmids and chromosomes of Methanoperedens, as well as other archaeal genomes. Borgs do not contain protein-coding genes that are associated with plasmids or viruses; they also lack rRNA genes, origins of replication, or other vital genes and features that are commonly found within minichromosomes found in archaea.
Borgs range from 0.66-0.92 Mbp in length, larger than the genomes of any known archaeal viruses. The sizes of Borg genomes are more characteristic of eukaryote-specific double-stranded DNA viruses from the phylum Nucleocytoviricota, also known as nucleocytoplasmic large DNA viruses, which can surpass 2.5 Mbp. Tandem direct repeat sequences are prevalent throughout Borg genomes, and they are terminated by inverted repeats. This differs from the megaplasmids of some bacteria, which carry interspaced repeats and usually are not responsible for encoding necessary genes.
Below is a list of genes known to be encoded by Borgs :