Researchers unravel a long-standing mystery about the unique hybrid structure found in bacteria

Review by Emily Henderson, Bachelor of ScienceNovember 5, 2020

A unique hybrid structure called letron, which is half RNA and half single-strand DNA, is found in many types of bacteria. Since its discovery about 35 years ago, researchers have learned how to use retorons to generate single strands of DNA in the laboratory, but despite much research on this issue, their function in bacteria. No one knew what was.

In the paper published today cellThe Weizmann Institute of Science team reports on solving a long-standing mystery. Retron is the “guard” of the immune system that guarantees the survival of bacterial colonies when infected with a virus. In addition to revealing new strategies that bacteria use to protect themselves from viral infections (surprisingly similar to those used in the plant’s immune system), this study will be added to the genome in the future. Revealed many new retorons that could be-editing toolkit.

The study, conducted in the laboratory of Professor Rotem Sorek of the Institute’s Department of Molecular Genetics, was conducted in his laboratory by Adi Millman, Dr. It was led by Aude Bernheim and Avigail Stokar-Avihail. Solek and his team did not try to solve the mystery of Retoron. They were looking for new elements of the bacterial immune system, especially those that help the bacteria fend off viral infections. Their search has been facilitated by the recent discovery that bacterial immune system genes tend to be concentrated in the genome within so-called defensive islands. When they discovered the unique characteristics of Retoron within the Bacterial Defense Island, the team decided to investigate further.

Their first study showed that this letron is definitely involved in protecting bacteria from viruses known as phages that specialize in bacterial infections. When researchers looked more closely at additional letrons near known defense genes, they found that letrons were always physically and functionally linked to one other gene. Bacteria were less successful in combating phage infection when either the associated gene or letron was mutated.

Researchers then set out to look for more such complexes in the Defense Islands. Ultimately, they identified about 5,000 retorons in different defensive islands of numerous bacterial species, many of which were new.

To see if these retorons generally function as an immune system, researchers transplanted many retorons one at a time into bacterial cells in the laboratory that lacked them. As they suspected, many of these cells have found retorons that protect the bacteria from bacteriophage infections.

How does Retoron do this? Focusing on a particular type of letron and tracking its effects in the face of phage infection, the researchers found that its function was to kill infected cells. Cellular suicide, once thought to belong only to multicellular organisms, is when the suicide mechanism works fast enough to kill cells before the virus makes a copy of itself and spreads to other cells. It is a last resort to stop widespread infections.

Further investigation has shown that Letron does not sense the phage invasion itself, but continues to monitor another part of the immune system known as RecBCD, one of the bacteria’s first lines of defense. If the phage becomes aware that it has tampered with the cell’s RecBCD, Retoron activates the program via a second linked gene, killing the infected cell and protecting the remaining colonies.

This is a clever strategy and has been found to work in the same way as the guard mechanism adopted by plant cells. Like viruses that infect plants, phages are equipped with different inhibitors to block different parts of the cell’s immune response. Like the guard mechanism known to be present in plants, retoron does not need to be able to identify all possible inhibitors just to process the function of one particular immune complex. Infected plant cells apply this “absorbent infection” method to kill small areas of leaves and roots to save the plant itself. Most bacteria live in colonies, so this same strategy can promote the survival of the group at the expense of individual members. “

Professor Rotem Solek, Faculty of Molecular Genetics, Weizmann Institute of Science

Retoron is very useful in biotechnology because it starts with a fragment of RNA that is a template for the synthesis of DNA strands. This template of retron sequences can be used in exchange for any DNA sequence and can be combined with CRISPR, another tool borrowed from the Bacterial Immune Toolkit, to manipulate genes in a variety of ways. Sorek and his team believe that in the diverse list of retorons they have identified, they may be hiding some lists that can provide templates that suit their specific genetic editing needs.