MAY 03, 2021 3:42 AM PDT

A New Bacterial Defense System is Discovered

WRITTEN BY: Carmen Leitch

Most DNA that we're familiar with is found in cells in a double-stranded form. So, many years ago, scientists were intrigued when they identified single-stranded DNA (ssDNA) throughout bacterial cells they were studying. They found that these ssDNA molecules were linked up to non-coding RNA molecules with complementary sequences and eventually identified an enzyme that was also part of this mysterious complex. When the enzyme in this complex was determined to be a reverse transcriptase, the complex was given the name retron. But no one knew what these retrons did or why.

Cropped from a Public domain image by Janice Haney Carr, National Escherichia, Shigella, Vibrio Reference Unit at CDC, USCDCP

Researchers eventually found that DNA sequences that code for parts of retrons are often accompanied by known bacterial defense genes. When there were mutations in those genes or the retrons, the bacteria wasn't as good at fighting off viral infection.

In a November 2020 report in Cell, scientists showed that once the retron is activated, the protein linked to the retron can zero in on the outer membrane of a bacterium and change how permeable it is. When the bacterium's first line of defense gets deactivated by a bacteriophage, the retron swings into action, using the protein to make the membrane permeable, killing the infected cell.

Related: Are Retrons the Next CRISPR?

"It's a clever strategy, and we found it works in a similar way to a guard mechanism employed in plant cells," said senior study author Professor Rotem Sorek of the Weizmann Institute of Science.

"Just like viruses that infect plants, phages come equipped with a variety of inhibitors to block assorted parts of the cell immune response. The retron, like a guard mechanism known to exist in plants, does not need to be able to identify all possible inhibitors, just to have a handle on the functioning of one particular immune complex. Infected plant cells apply this 'abortive infection' method, killing off a small region of a leaf or root, in an effort to save the plant itself. Since most bacteria live in colonies, this same strategy can promote the survival of the group, even at the expense of individual members."

Retrons don't keep watch for phages, but they monitor a bacterial immune system known as RecBCD. If a bacteriophage evades the RecBCD system, the retron is activated.

Retrons have something in common with a gene editor that's recently taken the biomedical research world by storm, CRISPR; they are both bacterial defense systems. CRISPR, however, targets and cuts genomic material, and it's still challenging to use it to integrate new sequences. But retrons can create a bunch of copies of a desired sequence and splice them into a cell's genome. The RNA in the retron can be switched out with another of any sequence, and used as a template for integration into the bacterial genome.

That work helped reveal a natural function for retrons, which are now known to utilize the non-coding RNA they carry as a template for the reverse transcriptase. Thus, that enzyme can generate a bunch of ssDNA molecules with that RNA template. Now that we know more about retrons, scientists are figuring out how to make them work for us.

Sources: Cell, Science

About the Author
  • Experienced research scientist and technical expert with authorships on over 30 peer-reviewed publications, traveler to over 70 countries, published photographer and internationally-exhibited painter, volunteer trained in disaster-response, CPR and DV counseling.
You May Also Like
SEP 07, 2021
Microbiology
Viral Ancestor of SARS-CoV May Date Back 22,000 Years
SEP 07, 2021
Viral Ancestor of SARS-CoV May Date Back 22,000 Years
The world has rapidly become familiar with sarbecoviruses, two of which jumped to humans in recent years. The first was ...
OCT 08, 2021
Microbiology
'Living Medicines' to Destroy Dangerous Biofilms
OCT 08, 2021
'Living Medicines' to Destroy Dangerous Biofilms
Bacteria that grow in colonies can be extremely difficult to eliminate; the bacteria in these groups often become resist ...
OCT 28, 2021
Genetics & Genomics
Research Suggests Bacteria & Fungi Interact Far More Than We Knew
OCT 28, 2021
Research Suggests Bacteria & Fungi Interact Far More Than We Knew
Genomic sequencing tools have enabled researchers to study microbial communities that are everywhere in our world, even ...
NOV 08, 2021
Coronavirus
Pfizer Claims Their New COVID-19 Pill Reduces Hospitalization Risk by 89%
NOV 08, 2021
Pfizer Claims Their New COVID-19 Pill Reduces Hospitalization Risk by 89%
Current treatments for COVID-19 have to be administered intravenously, so a pill that could treat patients would be huge ...
NOV 22, 2021
Plants & Animals
The Lungfish is a Living Antimicrobial Sponge
NOV 22, 2021
The Lungfish is a Living Antimicrobial Sponge
African lungfish are incredible animals that survive in freshwater or on land. Research on lungfish has taught us more a ...
NOV 26, 2021
Coronavirus
Heavily Mutated COVID-19 B.1.1.529 Variant Emerges, WHO Names It Omicron
NOV 26, 2021
Heavily Mutated COVID-19 B.1.1.529 Variant Emerges, WHO Names It Omicron
On Thursday, November 2021, South African health officials announced that they had identified a new variant of the pande ...
Loading Comments...