DNA is the genetic material used by all living things. But in some cases, the four DNA bases ̵1; adenine, thymidine, cytosine and guanine – have been chemically modified. And in viruses, things are more flexible, with many using RNA instead of DNA as the genetic material. In all of these cases, a fundamental pairing in the genetic material takes place in accordance with the rules first proposed by James Watson and Francis Crick.
So far, the only exceptions are viruses that infect bacteria and use their own seemingly unique bases. But researchers eventually examined it in more detail and found that this “Z-DNA” appeared to be used by dozens of viruses.
Not that Z
Confusingly, there is something else called Z-DNA. The DNA in our cells has a curve to the right called A-DNA to the double helix. But it is also possible to have a double strand with a left curve called Z-DNA.
but that DNA is not this DNA. This DNA is distinguished by bases, which are not found elsewhere, through other types of basal pairings. Diaminopurine, abbreviated as Z, is structurally similar to the adenine (A) found in normal DNA, but the diaminopurine has a special nitrogen hanging on one side, which causes it to form an additional hydrogen bond with thymidine (T), which. It’s a regular partner Thus, the ZT base pair attaches slightly more DNA double strands together than the standard AT pairing.
The DNA that combines Diaminopurine is called Z-DNA. We have known since 1979 that it exists in nature as a single virus called S-2L, which is infected with cyanobacteria. But until now, we don’t know whether the virus was a single bizarre thing, or it represented the tip of the biological iceberg that contains many viruses that we have yet to discover. Just as important, we’re not even sure how it ended up being included in the virus in the first place.
A large team of researchers, mostly from China, decide what happened here. They started by searching the S-2L virus genome to see if it encoded something wrong.
One type of gene present in the S-2L virus genome encodes a protein involved in the genes that cells use to make adenine, a base that is most similar to Z / diaminopurine. It was found that many amino acids involved in catalytic reactions were different. These changes affect what molecules can fit into the catalyst site on the protein encoded by the gene. A search of additional viral genomes showed that one of these specific changes was found in dozens of other viruses.
The researchers created viral proteins and incubated them with raw materials used by normal enzymes. They found that instead of forming an adenine precursor, the protein was a precursor to Z / diaminopurine. Another enzyme found in bacteria then converts to the mature Z-DNA base, so the virus has everything needed to make its own Z-DNA.
Looking at the dozens of viruses carrying a similar version of this gene, the researchers found that they were present in a genome that contained a few additional genes. One of these genes appears to be involved in making sure that there is enough chemical precursor to be converted to another Z / diaminopurine by simply removing all the phosphates connected to the adenine base. These phosphates are important for the use of the adenine bases to make DNA, so genes destroy the useful adenine group, essentially, hence the cell’s ability to make any DNA except Z-DNA.
Additionally, all of these genes are typically found in association with DNA polymerase, an enzyme that makes new copies of DNA, although this is not entirely clear. But it is likely that this enzyme has been modified to use Z / diaminopurine when DNA is copied.This could mean its resistance, as the enzyme mentioned in the last paragraph removes the beneficial adenine base. go Or they may wish to include Z / diaminopurine.
In any case, the researchers found more than 60 viral genomes containing combinations of these four genes. Obviously, Z-DNA is a typical aspect of viral life.
The obvious question is, why does life have to confront all these problems to have its own chemically different forms of DNA? The answer lies in how bacteria protect themselves from viruses. One of their main forms of defense is an enzyme that recognizes a specific sequence in the DNA and cuts it off. These bacteria chemically modify DNA in a way that it prevents it from being cut, meaning that the enzyme cuts only the foreign DNA as well as the virus.
It turns out that Z-DNA cannot be recognized with these cutting enzymes. Therefore, viruses completely avoid this kind of protection. The researchers tested several types of cutting enzymes and found that A usually had A at the target point, unable to cut. This indicates that the base of Z / diaminopurine Interferes with the enzyme’s ability to recognize any existing DNA for the virus researchers tested here.There’s no sign of any adenosine base, everything is Z-DNA.
In addition to showing that Z-DNA is more comprehensive than a single virus, there are many interesting effects. Perhaps the most important thing is to be identified. diaminopurine In the meteorite, which indicates that it can form on its own without too much trouble. The discovery was in line with the idea that some of the chemicals that sparked life could have arrived on Earth from space. However, it will raise the question of why. diaminopurine Therefore was subsequently replaced by adenine
In addition, DNA has also been used in many alternative forms.The ZT base pair, as we discussed above, should produce more stable interactions than the AT pair.This may be helpful in cases where researchers use DNA for structural purposes. Or calculation And the form of DNA that most proteins in cells cannot recognize immediately has many benefits.
Finally, this is only an interesting indication that, despite years of studies, But life still has surprises for us.
science, 2021 DOI: 10.1126 / science.abe4882 (About DOI)
List of images by Getty / Tek Image / Science Photo Library.