Transgenic Microorganisms

A transgenic microorganism is a microbe, usually a bacterium, into which genetic information has been introduced from the outside and which possesses the ability to pass that information on to subsequent generations in a stable manner. This is not an entirely novel idea in microorganisms, since bacteria have been practicing and perfecting this art over billions of years of evolution. We, on the other hand, have only recently learned to duplicate this phenomenon and turn it to our own purposes. Genetic engineering is the field that has developed as a consequence of research into this process. Its commercial application forms the basis of the biotechnology industry today.

Moving Genes between Species

The process by which scientists introduce new genetic material into a microorganism is called molecular or gene cloning. It involves the isolation of DNA from a source other than the microorganism itself. Source organisms span the world of living things, from microbes to plants to animals, including humans. Scientists obtain source DNA in several different ways: by disrupting cells of the target microbe (or plant or animal) and fragmenting it into small pieces, by synthesizing it from an RNA template using an enzyme called reverse transcriptase, or by knowing the specific gene sequence and synthesizing it directly in the laboratory.

Once obtained, the pieces of DNA are inserted into a small genetic component that has the ability to make copies of itself (replicate) independently from the microbial genome. This self-replicating unit is called a cloning vector. Although these genetic elements exist naturally in the form of plasmids and bacterial viruses, many of the ones used today have been altered to improve their properties for transferring genes. Restriction enzymes , which nick the donor DNA and the cloning vector at specific sites, and DNA ligase, which attaches the donor DNA to the cloning vector, allow the source genes of interest to be inserted into the cloning vector without disrupting its ability to replicate.

The next step in the process is the introduction of the cloning vector with its segment of new DNA into a living cell. Bacteria have the ability to transport DNA into their cells in a process called transformation, and this ability is commonly exploited to achieve this goal. Getting the DNA into the cell, however, is only the beginning. No transformation is 100 percent efficient, and so the bacteria that receive the gene(s) of interest must be separated from those that did not. One of the best studied and most commonly used cloning vectors, pBR322, is especially useful for this purpose, as it contains several genes for antibiotic resistance. Hence, any cell transformed with DNA containing pBR322 will be antibiotic resistant, and thus can be isolated from similar cells that have not be so transformed by merely growing them in the presence of the appropriate drugs. All that remains is to identify bacteria that are producing the product of the desired gene(s), and cloning is a success.

The introduction of human genes into bacteria has several complicating wrinkles that make cloning them even more challenging. For example, a bacterial gene codes for a protein from start to finish in one long string of nucleotides , whereas human cells have stretches of noncoding nucleotides called introns within their genes. Bacteria do not have the same ability as human cells to remove these introns when producing proteins from the gene, and if the introns are not removed, the intended protein cannot be produced. This, along with other complications, has been overcome using many of the tools of genetic engineering.

Commercial Application

Transgenic microbes have many commercial and practical applications, including the production of mammalian products. A company called Genentech was among the earliest and most successful commercial enterprises to use genetically engineered bacteria to produce human proteins. Their first product was human insulin produced by genetically engineered Escherichia coli. A variety of other human hormones , blood proteins, and immune modulators are now produced in a similar fashion, in addition to vaccines for such infectious agents as hepatitis B virus and measles.

Another promising application of genetically engineered microbes is in environmental cleanup, or biomediation. Scientists have discovered many naturally occurring genes that code for enzymes that degrade toxic wastes and wastewater pollutants in bacteria. Examples include genes for degrading chlorinated pesticides, chlorobenzenes, naphthalene, toluene, anilines, and various hydrocarbons. Researchers are using molecular cloning to introduce these genes from several different microbes into a single microbe, creating "super microbes" with the ability to degrade multiple contaminants.

Ananda Chakrabarty created one of the first microbes of this nature in the early 1970s. He introduced genes from several different bacteria into a strain of Burkholderia cepacia, giving it the ability to degrade toxic compounds found in petroleum. This microbe offered a potential alternative to skimming and absorbing spilled oil. Chakrabarty's genetically modified bacterium has never been used, however, due to public concerns about the release of genetically engineered microbes into the environment. The microbe did, on the other hand, play an important role in establishing the biotechnology industry. The U.S. Patent Office granted Chakrabarty the first patent ever for the construction and use of a genetically engineered bacterium. This established a precedent allowing biotechnology companies to protect their "inventions" in the same way chemical and pharmaceutical companies have done in the past.

see also Bioremediation; Escherichia coli (E. coli bacterium); Gene; Plasmid; Reverse Transcriptase; Transformation; Transgenic Animals; Transgenic Organisms: Ethical Issues; Transgenic Plants.

Cynthia A. Needham

Bibliography

Glick, Bernard R., and Jack J. Pasternak. Molecular Biotechnology: Principles and Applications of Recombinant DNA, 2nd ed. Washington, DC: ASM Press, 1998.

Madigan, Michael T., John M. Martinko, and Jack Parker. Brock Biology of Microorganisms, 9th ed. Upper Saddle River, NJ: Prentice Hall, 2000.

Needham, Cynthia A., Mahlon Hoagland, Kenneth McPherson, and Bert Dodson. Intimate Strangers: Unseen Life on Earth. Washington, DC: ASM Press, 2000.

Snyder, Larry, and Wendy Champness. Molecular Genetics of Bacteria. Washington, DC: ASM Press, 1997.