Ribosomes were imprisoned for self-reproduction

Daria Spasskaya, N+1

Biochemists from Harvard have built a mathematical model that connects the structure of the ribosome with the rate of doubling of this organelle. It turned out that the composition of ribosomes in bacteria corresponds to the principle of the fastest self-reproduction, which allows bacteria to divide quickly. Mitochondrial ribosomes, on the contrary, are not involved in self-reproduction, so they differ significantly in composition from bacterial ones. An article with calculations was published in the journal Nature (Reuveni et al., Ribosomes are optimized for autocatalytic production).

Ribosomes are cellular organelles that synthesize protein by translating the genetic code of matrix RNAs into amino acids. In all living organisms, ribosomes are fundamentally arranged in the same way – several large ribosomal RNA molecules act as a backbone on which many small ribosomal proteins are located. Despite the unified structure and principle of operation, in bacteria, archaea and eukaryotes, ribosomes differ in size, number and mass of components. For example, the bacterial ribosome is smaller in size, contains three RNA molecules and an average of 55 different proteins. The eukaryotic ribosome is larger than the bacterial one, contains four RNA molecules and 80 proteins.

The speed of ribosomes is a very important factor that ultimately determines the time of cell doubling, because the ribosome produces literally all protein components. The reproduction of ribosomes also falls on the shoulders of other ribosomes – the exception is ribosomes, which are part of the mitochondria. For them, structural proteins are synthesized by organelles located in the cytoplasm of the "host" cell. Scientists have estimated that the ribosome spends 20 percent of all "working time" on self-replication, that is, the synthesis of ribosomal proteins, as well as RNA polymerase molecules, which synthesizes new ribosomal RNA molecules.

Betting on RNA allows you to increase the rate of reproduction of ribosomes. The graph shows that with an increase in the protein component, the rate of generation of new ribosomes decreases (figures are from an article in Nature).

Molecular biologists have suggested that the high speed of work, and hence self-reproduction, should affect the composition of organelles. For example, the synthesis time of individual components will be shorter if the ribosome contains many short proteins, rather than one or two long ones. The authors calculated that the optimal number of proteins, allowing the ribosome to be assembled as quickly as possible, lies in the range from 40 to 85. Indeed, for bacteria, this value is 55. Eukaryotic ribosomes with 80 proteins are already close to the upper limit of efficiency, beyond which factors such as impaired equimolarity of components and low efficiency of translation initiation begin to reduce the rate of reproduction.

The average length of a ribosomal protein in bacteria is 130 amino acids, while the average length of a polypeptide in a cell as a whole is more than 300 amino acids. Shortened proteins not only allow their synthesis to be completed quickly, but also eliminate errors that occur during translation. As for ribosomal RNA, according to the model, it is more profitable for a cell to synthesize one large molecule, rather than many small ones – so it is easier to assemble a ribosome. In reality, ribosomes consist of several RNA molecules, but the authors note that in bacteria, for example, they are synthesized in the form of one large precursor, which is subsequently cut.

Since RNA is easier and faster to synthesize than proteins, for the rapid self-reproduction of the ribosome, RNA should make up most of it. Indeed, in the most "high–speed" bacterial ribosomes, RNA is 70-80 percent, and in the composition of mitochondrial ribosomes - only 20 percent.

The composition of ribosomes reflects their ability to reproduce themselves. Bacterial and eukaryotic ribosomes are less than half made up of proteins (blue color), and mitochondrial consists of 80 percent protein.

Earlier we wrote that the reproduction of ribosomes imposes restrictions not only on the rate of cell division, but also on its size. For example, if a bacterial cell grows in size while maintaining the same rate of division, pretty soon it simply will not be able to accommodate all the ribosomes it needs. The new work demonstrates that the increase in the size and complexity of the organization of cells led to the evolution of ribosomes, which also became larger and more complex, but at the same time lost speed.

Portal "Eternal youth" http://vechnayamolodost.ru  25.07.2017