Scientists Direct Bacteria with an Expanded Genetic Code to Develop Extreme Heat Tolerance

November 28, 2018 ( Nanowerk News ) In recent years, scientists have developed bacteria with expanded genetic codes producing proteins from a more large number of molecular building blocks, thus opening up a promising frontier in protein engineering. Now scientists at Scripps Research have shown that such synthetic bacteria can develop proteins in the laboratory with improved properties using mechanisms that might not be possible with nature's 20 amino acid building blocks. By exposing bacteria with an artificially expanded genetic code to temperatures at which they normally can not grow, the researchers found that some of these bacteria formed new heat-resistant proteins that remained stable at temperatures where they would be present. normally inactivated. The researchers reported their findings in the Journal of the American Chemical Society ("Improving protein stability with genetically encoded non-canonical amino acids"). Virtually all organisms on Earth use the same 20 amino acids as the building blocks of protein production – the large molecules that provide the majority of cellular functions. Peter Schultz, PhD, lead author of JACS and President and CEO of Scripps Research, has developed a method to reprogram the protein biosynthetic mechanism of the cell to add new amino acids to proteins, called non-canonical amino acids (ACNA), with structures and chemical properties not found in the 20 common amino acids. This extended genetic code has been used in the past to rationally design new-valued proteins, to be used as tools to study the function of proteins in cells, and as new, high-precision anticancer drugs. Researchers have now asked whether synthetic bacteria with extended genetic codes have an evolutionary advantage over those limited to 20 constituents: is a 21 amino acid code better than a 20 amino acid code from the point view of the evolutionary form? "Since we have expanded our range of amino acids that can be incorporated into proteins, many studies have been devoted to the use of these systems for the manufacture of molecules with new or improved properties" says Schultz. "Here we have shown that the combination of an extended genetic code with a laboratory evolution allows for the creation of proteins with improved properties that might not be easily achievable with the more limited set of nature." Scientists began tweaking the E. coli genome so that bacteria could produce the homoserine o-succinyltransferase (metA) protein using a 21 amino acid code instead of the 20 amino acid common code. Meta, an important metabolic enzyme, determines the maximum temperature at which E. coli can thrive. Above this temperature, the metA begins to inactivate and the bacteria die. The researchers then made meta mutants, in which almost all the amino acids of the natural protein could be replaced by a 21st non-canonical amino acid. At this point, they let natural selection – the central mechanism of evolution – operate its magic. By heating the bacteria to 44 degrees Celsius – a temperature at which the normal meta protein can not work and, as a result, the bacteria can not grow – scientists exert a selective pressure on the bacteria population. As expected, some of the mutant bacteria were able to survive beyond their usual temperature cap, thanks to the possession of a heat-stable mutant metA. All other bacteria are dead. In this way, the researchers were able to get the bacterium to develop a mutant enzyme, the metA, able to withstand temperatures 21 degrees higher than normal, nearly double the increase in stability thermal that people usually get when they are limited to mutations limited to normal. building blocks of amino acids. The researchers then identified the genetic sequence change specific to the origin of the mutant metA and found that it was due to the unique chemical properties of one of their non-canonical amino acids that evolution in the laboratory was smartly exploiting to stabilize the protein. "It is striking that such a mutation with a new amino acid not present in nature leads to a significant improvement in the physical properties of the protein," Schultz says. "This experiment raises the question of whether a 20-amino acid code is the optimal genetic code.If we discover life forms with extended codes, will they have an advantage in terms of the number of amino acids? evolution, and what would it be if God had worked on the seventh day and added a few amino acids to the code? "