The first successful instance of genetic modification came in 1973 when Herbert Boyer and Stanley Cohen successfully modified an E. coli to produce a Salmonella gene. Since then, the area of genetic engineering has taken off, producing new and different genetically modified organisms every year.
Even with all of these advances, the end goal has never been reached: the creation of an entirely new and completely engineered organism via the complete overhaul of an existing species genome. Recently, however, researchers at the Harvard Medical School came one step closer to the end goal by successfully recoding the E. coli MG1655 genome.
An organism’s genome, consisting of mostly DNA and RNA, contains all of the organisms’ hereditary information. DNA and RNA sequences are comprised of sequences of four nucleotides; adenine, guanine, cytosine and thymine (In DNA) or uracil (in RNA). These nucleotides are arranged in a specific order. These specific arrangements create different genes, which code for specified proteins.
The gene is first converted into a messenger RNA before entering the cytoplasm of the cell. The process of converting the copied DNA sequence to a protein is completed by the ribosome in a cell’s cytoplasm. Every three nucleotides in the gene translate for a specific amino acid, which are the building blocks for proteins. Subsequent amino acids are strung together by the ribosome to create proteins. The arrangement of the codons and the nucleotides is of utmost importance because if there are any errors or missing codons, the resulting protein will be dysfunctional.
This study included two different experiments. The first experiment sought to answer the question as to whether or not it is possible to replace one codon with another, thereby changing the protein the gene codes for. First, researchers removed the release factor 1 gene (which codes for RF1 that terminates gene translation at UAG sequences) as well as all UAG codons and in the E. coli MG1655 genome. The UAG codon is one of the “stop codons” which acts as a signal for the termination of translation. In the deleted gene locations, the researchers put UAA codons.
The deletion of RF1 was important as it ensured that any genes that were horizontally transferred to the recoded E. coli would be dysfunctional. The resultant E. coli from this experiment were fully functional and actually showed a 60% increase in their doubling time. The UAG sequence was chosen because it is the rarest genome in the MG1655 genome, occurring only 321 times.
The second part of the experiment involved the replacement of 13 different codons across 42 separate E. coli genes with a codon that translated into the same amino acid (for example, replacing CGU for CGC which both correspond to the amino acid, arginine). In the end, 24% of DNA across the 42 genes targeted were changed. However, the proteins that were coded were completely functional and replicas of the normal protein.
The ability to target specific sequences for deletion and modification that this research introduces will have an enormous impact on the tools available for the modification of organisms. Furthering this research will open new doors and bring scientists closer to the holy grail of bioengineering; creation of a novel, completely engineered organism.
