Engineered Bacteria Are Genetic Rebels
Goho, A., Science News
In an ongoing effort to push the limits of genetic engineering, researchers have created a bacterium that can incorporate artificial amino acids into its proteins and do so by breaking a fundamental rule of molecular biology.
Virtually all organisms build their life-sustaining proteins from a set of 20 amino acids, as encoded in an organism's DNA. The genetic code is represented by sequences of four types of nucleotides, designated by the letters A, T, G, and C. Those sequences are broken down into three-letter blocks called codons. The cell's molecular machinery translates each codon into an amino acid and strings those amino acids together to form proteins.
To create an organism capable of making proteins from amino acids beyond the basic set of 20, a team of researchers led by Peter Schultz at the Scripps Research Institute in La Jolla, Calif., decided to expand the genetic code. Instead of having a three-letter codon system, the researchers created a four-letter system.
That "opens the possibility of adding multiple unnatural amino acids to the genetic code," says Christopher Anderson, one of the Scripps members who developed the organism.
Previously, the Schultz lab tricked bacteria and yeast cells into translating a naturally occurring three-letter nonsense codon--one that normally has no associated amino acid--within the cells' genomes into one of several artificial amino acids (SN: 8/16/03, p. 102). Although that enabled the organisms to churn out proteins using one extra amino acid, moving beyond 21 amino acids in this way would require the addition of new, unique codons, says Anderson.
In this latest experiment, the researchers engineered the gene-to-protein translation machinery of Escherichia coil cells to recognize the four-letter genetic sequence AGGA and to assign the artificial amino acid L-homoglutamine to that codon. …