The magic word for bacteria
Rare codons help regulate the movement of ribosomes through mRNA
Usually, such molecular biotechnological manipulations are reduced to the fact that a gene encoding the desired protein is inserted into the genome of a bacterium; there may be several such genes, and these proteins may have very different properties. However, the synthesis of mRNA on DNA and the subsequent synthesis of a protein molecule on mRNA are subject to many factors that, of course, affect the activity of all this machinery. And the need to take them into account is a constant headache for those who are engaged in such molecular genetic work.
One of these factors is associated with rare codons – triplets of nucleotides corresponding to certain amino acids. As is known, all amino acids used in protein synthesis are encoded in the genetic code by "words" of three nucleotide letters; however, there are much more such "words" in the code than amino acids, that is, it turns out that more than one codon corresponds to the same amino acid. These codons are used in genes with different frequencies, some more often, others less often; the latter are therefore called rare.
Some time ago, researchers noticed that in bacteria such rare codons tend to the beginning of the coding region in the gene, and on the mRNA, the ribosome, therefore, encounters them first. Moreover, the more rare codons appeared at the beginning, the more protein was synthesized on such a matrix. No one knew why this was happening, but the assumptions were very different. According to one hypothesis, rare codons serve as brakes on ribosomes: on such codons, the ribosome has to wait for an aminoacylated transport RNA with an amino acid corresponding to the codon to come to it. Then, on ordinary codons, the ribosome gradually accelerates. If there are no rare codons at the beginning, then the ribosomes immediately accelerate, and it happens that the one coming from behind catches up with the front one, collides with it, and this accident stops biosynthesis. And if there are rare codons at the beginning of the mRNA, then they, as speed regulators, make it so that all ribosomes get to the end of the mRNA, thereby increasing protein production.
According to other assumptions, it turned out that rare codons somehow change the spatial arrangement of mRNA, but these changes again affect the speed of ribosome movement.
Three researchers from the Weiss Institute at Harvard University (USA) experimentally tried to test these hypotheses. First, they found out how much rare codons increase protein production. To do this, rare and common codons were inserted into a green fluorescent protein that was injected into the bacterium. By the way the bacterium glowed, it was possible to understand how the initial codons work.
According to the authors of the paper in Science (Goodman et al., Causes and Effects of N-Terminal Codon Bias in Bacterial Genes), the appearance of only one rare codon could enhance protein synthesis by 60 times.
Secondly, the researchers compared the speed and efficiency of protein synthesis on mRNA with rare codons and on mRNA without rare codons, but with a spatial structure that slows down ribosomes. As a result, it turned out that both really increase the efficiency of synthesis, but rare codons work by themselves and their effect does not depend on the structure of mRNA.
The fundamental and practical conclusions from the results are obvious: it was possible not only to experimentally confirm the hypothesis concerning one of the most common problems in molecular biology, but also to show by what tricks it is possible to force bacteria to produce more biotechnological product.
Based on the materials of the Harvard Gazette: Programming genetic code can lead to better designer genes.
Portal "Eternal youth" http://vechnayamolodost.ru30.09.2013