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Scripps Research scientists and their collaborators have created microorganisms that can summarize the essential characteristics of organisms thought to have lived billions of years ago, allowing them to explore how life has evolved from from inanimate molecules to unicellular organisms, via the complex. multicellular life forms that we see today
By studying one of these modified organisms – a bacterium whose genome is composed of both ribonucleic acid (RNA) and DNA. deoxyribonucleic acid (DNA) – scientists hope to shed light on the early evolution of genetic material,
Using a second artificial organism, a genetically modified yeast containing an endosymbiotic bacterium, they hope to better understand the origins of cellular plants called mitochondria. Mitochondria provide essential energy for eukaryotic cells, a large group of organisms – including humans – with complex cells containing a nucleus.
Researchers report the engineering of microbes in two articles, one published October 29, 2018 in the Proceedings of the National Academy of Sciences (PNAS) and other published August 30 2018 in the Journal of the American Chemical Society (JACS).
"These modified organisms will allow We will explore two key theories about the major stages in the evolution of living organisms: the transition from the world of RNA to the world of DNA and the transition of prokaryotes to eukaryotes with mitochondria, "said Peter Schultz, senior author of the publications and president of Scripps. Research. "Access to easily manipulated laboratory models allows us to search for answers to questions about early evolution that were previously intractable."
The origins of life on Earth have fascinated mankind for millennia. Scientists have traced the arc of life for several billion years and concluded that the simplest forms of life emerged from the primordial chemical soup of the Earth and then changed over the centuries into more and more complex organisms.
A monumental leap came with the emergence of DNA, a molecule that stores all the information necessary for replication of life, commanding cellular machines to meet their needs by generating mainly the most important components. RNA, which in turn directs the synthesis of proteins, the molecular battlehorses of cells.
In the 1960s, Carl Woese and Leslie Orgel, as well as Francis Crick, pioneer of DNA, argued that before, organisms depended on RNA for transmitting genetic information, a similar molecule but much less stable than DNA, which can also catalyze chemical reactions such as proteins.
"In science clbad, students learn that DNA leads to RNA, which in turn leads to proteins – it's a central dogma of biology – on its head," said Angad Mehta , first author of the new articles and postdoctoral research badistant at Scripps Research. "For the RNA world hypothesis to be true, you have to go from RNA to the DNA genome, but the question of how that could have happened is still a very big question. for scientists. "
One possibility is that the transition is made by the type of microbial missing link, an organism that replicates itself and stores genetic information in the form of RNA.
For the study JACS the team led by Scripps Research created Escherichia coli a bacterium that partially constructs their DNA with ribonucleotides, building blocks Moleculars generally used to build RNA. These manipulated genomes contained up to 50% RNA, which simultaneously represents a new type of synthetic organism and may be backtracked billions of years ago.
Mehta warns that their work has hitherto focused on the characterization of this chimeric DNA-RNA genome. its effect on bacterial growth and replication, but has not explicitly explored the issues on the transition from the world of RNA to the world of DNA. But the fact that E. coli with half of its genome made up of RNA can survive and replicate itself is remarkable and seems to support the possibility of the existence of organisms in transition evolutionary with hybrid RNA-DNA genomes.
The Scripps research team is currently studying how genomes of their genetic engineering E. coli and plans to use the bacterium to explore a number of issues relating to the disease. ;evolution.
For example, one may wonder whether the presence of RNA leads to rapid genetic drift – large changes in the gene sequence in a population over time. Scientists badume that mbadive genetic drift occurred early in the evolutionary stage and that the presence of RNA in the genome may help explain how quickly genetic change occurs. 39 is produced.
In the document published in PNAS researchers report an engineering. another laboratory model for an evolutionary milestone that would have occurred more than 1.5 billion years ago. They have created a yeast whose energy depends on the bacteria that live there and which constitutes a beneficial parasite or "endosymbionte". This composite organism will allow them to study the ancient origins of mitochondria – tiny organelles similar to bacteria that produce chemical energy in higher organism cells.
Mitochondria are generally thought to have evolved from ordinary bacteria captured by larger single-celled organisms. They fulfill several key functions in the cells. More importantly, they serve as oxygen reactors, using O 2 to form the basic unit of chemical energy of cells, the ATP molecule.
As crucial as mitochondria are for cells, their origins remain somewhat mysterious, though
Mitochondria have a double membrane structure like that of some bacteria and – again, like bacteria – contain their own DNA. The mitochondrial genome badyzes suggest that he shares an ancestor with the modern bacterium Rickettsia that can live in the cells of their hosts and cause disease. Stronger support for the bacterial origin of the mitochondria theory would come from experiments showing that independent bacteria could actually be transformed, in a evolution similar to that of an evolution, into symbiots similar to those mitochondria.
To this end, scientists at Scripps Research designed . E. coli bacterium that could live in, depend on the cells of Saccharomyces cerevisiae also called baker's yeast, and provide essential badistance to these cells.
The researchers began by modifying E. coli does not have the gene encoding thiamine, which makes the bacteria dependent on yeast cells for this essential vitamin. At the same time, they added to the bacterium a gene for ADP / ATP translocase, a transporter protein, so that the ATP produced in the bacterial cells is provided to their yeast cell hosts, mimicking the central function real mitochondria. [19659002] The team also modified the yeast so that its own mitochondria do not provide enough ATP. Thus, the yeast would depend on the bacterium for the normal production of ATP based mitochondria.
The team discovered that some of the manipulated bacteria, after being modified with surface proteins to protect them from destruction by yeast, were living and proliferating. in harmony with their hosts for more than 40 generations and seemed to be viable indefinitely.
"Modified bacteria appear to accumulate new mutations within the yeast to better adapt to their new environment," said Lubica Supekova, co-author of the document PNAS and a scientist at Scripps Research
. Once this system is in place, the team will attempt to evolve E. coli to become organelles similar to mitochondria. For the new E. colos endosymbionte, adapting to life in yeast could allow him to lose weight radically his genome. A typical example E. The bacterium coli for example, has several thousand genes, while the mitochondria have developed a reduced set of 37 cells only.
The Scripps research team completed the study with further gene subtraction experiments, and the results were promising. : they discovered that they could eliminate not only the E. coli thiamine gene, but also the genes underlying the production of the NAD metabolic molecule and the serine d & # 39; 39, amino acid, and still get a viable symbiosis.
"We are now on the right track to show that we can remove genes to make all 20 amino acids, which are an important part of the E. coli genome," said Schultz . "Once we reach this goal, we will remove genes for the synthesis of cofactors and nucleotides. In a few years, we hope to be able to get a truly minimal endosymbiotic genome. "
Researchers also hope to use similar endosymbiotic systems to study other important episodes of evolution, such as the Origin of chloroplasts, light-absorbing organelles playing a role similar to that of mitochondria in the energy supply of plants.
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