Redesigning Bacteria

The development of the recombinant DNA technique ushered in a new era of genetic engineering--with all of its promise and possible peril. The lowly organism that currently plays the largest role in the process is the E. coli bacterium. This microbe--a laboratory derivative of a common inhabitant of the human intestine--lends itself to being engineered because its genetic structure has been so well studied. In the first step of the process, scientists place the bacterium in a test tube with a detergent-like liquid. This dissolves the microbe's outer membrane, causing its DNA strands to spill out in a disorderly tangle. Most of the DNA is included in the bacterium's chromosome, in the form of a long strand containing thousands of genes. The remainder is found in several tiny, closed loops called plasmids, which have only a few genes each and are the most popular vehicles for the recombinant technique.

After the plasmids are separated from the chromosomal DNA in a centrifuge, they are placed in a solution with a chemical catalyst called a restriction enzyme. This enzyme cuts through the plasmids' DNA strips at specific points. It leaves overlapping, mortise-type breaks with "sticky" ends. The opened plasmid loops are then mixed in a solution with genes--also removed by the use of restriction enzymes--from the DNA of a plant, animal, bacterium or virus. In the solution is another enzyme called a DNA ligase, which cements the foreign gene into place in the opening of the plasmids. The result of these unions are new loops of DNA called plasmid chimeras because, like the Chimera--the mythical lion-goat-serpent after which they are named--they contain the components of more than one organism.

Finally, the chimeras are placed in a solution of cold calcium chloride containing normal E. coli bacteria. When the solution is suddenly heated, the membranes of the E. coli become permeable, allowing the plasmid chimeras to pass through and become part of the microbes' new genetic structure. When the E. coli reproduce, they create carbon copies of themselves, new plasmids --and DNA sequences--and all. Thus they become forms of life potentially different from what they had been before--imbued with characteristics dictated not only by their own E. coli genes but also by genes from an entirely different species.

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