The world's first artificial single-chromosome eukaryotic cell was born

Yan Zhongjun (second from left) The research team is analyzing the pulse field gel electrophoresis verification map of the artificial yeast strain.

Yang Zhengxing

Recently, the Institute of Excellence in Molecular and Plant Science of the Chinese Academy of Sciences/Key Laboratory of Synthetic Biology of the Institute of Plant Physiology and Ecology, the research team and collaborators of the Institute of Plant Physiology and Ecology, have worked hard for four years to create a single chromosome of eukaryotic cells for the first time in the world. A major milestone in biology.

The result was published online on August 2nd, Beijing time in the internationally renowned academic journal Nature.

Boldly guess, can you load all genetic material with one linear chromosome?

Can humans create life? In 2010, American scientist Klein Venter and his research team reported the world’s first “artificial life” in Science, which contains almost all the chemical synthesis and almost identical to the natural chromosome sequence. The prokaryotic mycoplasma caused a sensation.

This time, the research team based on the research team of Sui Zhongjun completed the creation of a single chromosome of 16 cells with a single cell eukaryote, Saccharomyces cerevisiae, as a complete function. This work shows that natural and complex life systems can be reduced by human intervention, and the boundaries of natural life can be artificially broken, and even artificially create new life that does not exist in nature.

In the long evolutionary process of nature, different living organisms have gradually formed their own unique genomes, including relatively stable DNA sequences and fixed chromosome numbers. The chromosome carries genetic information about the growth and reproduction of living organisms. Biological textbooks divide living organisms that exist in nature into prokaryotes and eukaryotes. Prokaryotic organisms with a chromosomal naked nuclear membrane usually contain a chromosome of a circular structure, and eukaryotes whose nucleus is surrounded by a nuclear membrane usually contain multiple chromosomes of a linear structure.

“The genome of eukaryotes is scattered on multiple chromosomes, and the number of chromosomes varies from species to species. For example, humans have 23 pairs of chromosomes, mice have 20 pairs of chromosomes, and fruit flies have only 4 pairs of chromosomes. How are these differences caused? Is the number related to the function of the organism? What are the advantages of multiple chromosomes relative to a single chromosome?” Yan Zhongjun said, “I think, can we break the boundary between prokaryotes and eukaryotes, and then artificially create a single chromosome? Creatures have normal functions?"

Therefore, Researcher Yan Zhongjun boldly assumed that eukaryotes can also load all genetic material and complete normal cell functions with a linear chromosome like prokaryotes.

15 rounds of fusion to create a single-chromosome yeast strain with gene editing technology

After the bold conjecture, Yan Zhongjun began to lead the team to conduct a series of experiments. They decided to use Saccharomyces cerevisiae as experimental material

Sui Shoujun said: "Saccharomyces cerevisiae is a single-cell eukaryote with 16 chromosomes and is a typical multi-chromosome eukaryote. It is a model material for basic research in molecular biology. Scientists all over the world have studied half of it. Century, very classic. In addition, yeast research background is clearer and easier to operate than most eukaryotes, which makes our work relatively simple."

In addition, published results show that as of March 2017, six chromosomes of Saccharomyces cerevisiae have been artificially synthesized.

Yan Zhongjun said: "On this basis, I am more determined by the above ideas. Can we go one step further and rearrange the chromosomes of the whole Saccharomyces cerevisiae?" Subsequently, he and Associate Professor Xue Xiaoli "engineered precision design" customized man-made The guiding principle of single-chromosome yeast and the overall scheme of rational analysis, experimental design, and engineering advancement. In 2013, the fusion of yeast chromosomes was officially launched.

So how do you achieve the fusion of two chromosomes?

"There is a complete eukaryotic line chromosome, usually containing one centromere for chromosome separation and two telomeres for protecting the ends of chromosomes. To achieve fusion of two chromosomes, you need to have two The two telomeres of the chromosome are removed and connected to each other. At the same time, one centromere of the two chromosomes needs to be removed, thereby ensuring the normal separation of the chromosome during cell division."

According to Zhai Zhongjun, in this process, it is particularly important to remove both telomeres and one centromere at the same time. Otherwise, it will be unstable and will break immediately. The CRISPR-Cas9 editing technology helped the research team do the job well. "It can connect two chromosomes and reorganize it very efficiently, which means that it can be done simultaneously and simultaneously."

In addition, the order of chromosome fusion is random. The research team conducted a series of validation experiments before doing chromosome fusion. The results showed that 8 pairs of chromosomes were successfully randomized, and the resulting strain grew robustly like the wild-type yeast strain.

Next, using the same chromosome fusion method, the research team carried out 15 rounds of chromosome fusion, and finally successfully created a yeast SY14 strain with only one linear chromosome.

Artificially modified yeast cells are robust and do not exhibit significant growth defects

After the successful creation of the SY14 strain, the team of the 覃重军 further cooperated with the Zhao Guoping research group of the Key Laboratory of Synthetic Biology, the researcher of the Zhou Jinqiu Researcher of the Institute of Biochemistry and Cell Biology of the Chinese Academy of Sciences, the researcher of Wuhan Fraser Gene Information Co., Ltd. and the Institute of Military Medical Sciences, Zhao Zhihu, etc. Teamwork, in-depth identification of the metabolic, physiological and reproductive functions of SY14 and its three-dimensional structure of chromosomes.

The results showed that the single-chromosome yeast showed almost the same transcriptome and phenotype profile as the wild type, but the progeny were sexually propagated by meiosis. In addition, the most obvious change that exists after chromosome fusion is the three-dimensional structure of the chromosome.

"Although the fusion significantly changes the three-dimensional chromosome structure, in addition to deleting a few non-essential genes, the new strain contains the same genetic material as normal Saccharomyces cerevisiae." 覃重军 said, "It has been proven that artificially modified yeast cells are unexpectedly robust. There are no significant growth defects under different culture conditions. However, fusion chromosomal strains do exhibit small adaptive limitations and sexual reproductive defects, so they may be rapidly eliminated by natural strains. These new findings It will also help explain the advantages of having more chromosomes."

Han Bing, academician of the Chinese Academy of Sciences and director of the Center for Excellence in Molecular Plant Science of the Chinese Academy of Sciences/Institute of Plant Physiology and Ecology, said that the natural and complex yeast chromosomes were expressed in a new and simplified form through artificial transformation, following the "artificial life" of prokaryotic bacteria. A major breakthrough has opened up a new direction for human beings to study the nature of life.

Paul Evans, director of the China Institute of Natural Sciences, said that these yeast strains can also be a powerful resource for studying the basic concepts of chromosome biology, including chromosome replication, recombination and separation, which are important in the biological field for a long time. theme.

It is worth mentioning that Saccharomyces cerevisiae is usually an important model for studying chromosomal abnormalities, and its 1/3 gene is homologous to a human gene with 23 pairs of chromosomes. The single linear chromosome yeast created by the 覃 heavy army team will provide models for many research topics in the future. At the same time, it can also provide a model for studying human telomere function and cell senescence.

"The telomere is the protective structure at the end of the linear chromosome, and the premature aging of humans is directly related to the telomere length of the chromosome. As the number of cell divisions increases, the length of telomeres gradually decreases, when telomeres become impossible. In a short time, the cells will die. In addition, the shortening of telomeres is also associated with many diseases, including tumor formation."

People's Daily (12th edition, August 3, 2018)

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