The sequel: What is the immune system?
In the previous part, we learned a lot about the immune system and ended on the wondering note of bacteria fighting off bacteriophages, correct? So the main question arose behind the mechanisms of how exactly they protect their own DNA from being degraded as viral DNA? Let’s get into the details of this and expand our knowledge!
So, nature and bacteria found a way. How?
Well, put shortly – they protected their DNA by chemically altering it.
And the longer version goes something like this.
Bacteria’s favorite way is to mark their DNA with one carbon atom (methyl group).
The strategy was ingenious – their DNA is protected, and they use enzymes like lasers to cut the viral DNA. In this way, if you cut a virus’s DNA – you would kill it.
As such enzymes cause restriction for the virus to attack the bacteria, the enzymes are called restriction enzymes.
But the strategy could not last forever, the inconvenient notion of an evolutionary race got its fingers tangled.
Some bacteriophages have found a way to protect their DNA. The T2 bacteriophage that Mary Human was working with found a way to chemically alter its DNA.
After entering the bacterium, the DNA of the virus changed in a way that would encourage the bacterium to bind sugar to the DNA. This would protect them from the attack of restriction enzymes and the bacteriophage would be able to attack the bacteria.
By a stroke of luck, the Escherichia coli used by Mary Human and Salvador Luria had a “genetic defect”.
Namely, it did not have a compound that would allow sugar to bind to viral DNA.
Upon entering the bacterium, the bacteriophage would release its DNA, but since Escherichia coli could not bind sugar to it, the DNA of the T2 bacteriophage would remain unprotected, and the bacteriophage could not grow further.
But when such a bacteriophage was transferred to another species of Shigella dysenteriae, that species could bind the sugar to the DNA of the bacteriophage and it would become lethal again.
Salvador Luria used very sympathetic terms – a “sweet” bacteriophage was one that was protected from the bacteria’s defense system, and a “sour” one was one that was sensitive.
When they published their work in 1952, the long race for the enzymes responsible for it had just begun. At that time, it was not even known that by methylating bacteria, they protect their own DNA from enzymes that cut DNA like scissors.
In the beginning of the 70s, Hamilton Smith and his team were the first to isolate a restriction enzyme from the bacterium Haemophilus influenzae. They called it Hind II.
Soon after, there was an explosion of discoveries and hundreds of enzymes were discovered that cut precisely specific sequences.
Soon, restriction enzymes became the scissors not of bacteria, but of biotechnology and molecular biology.
Scientists started using those scissors to cut large DNA molecules into smaller fragments, analyze them and use them if they want to create a protein in laboratory conditions.
They have become one of the most important tools not only of biotechnology, but also of the production of biotechnological drugs such as insulin, as well as diagnostics.
Restriction enzymes deserve an important place in the history of science.
They also helped us move the concept of the immune system far beyond the human one.
Our immune system is only a small part of a much bigger and much older story!
Man is not the center of the Earth, let alone the universe; we are neither the first nor the last.
