Maize Research Leaps Ahead with Early Jumping Gene Discoveries
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| Geneticists Donald S. Robertson and Peter A Peterson |
Two premier researchers serving on the faculty at Iowa State University for 50 years are pioneering contributors to the field of maize cytogenetics. Research discoveries through the course of their careers in the area of transposon biology by Peter A. Peterson, professor in the Department of Agronomy and Donald S. Robertson, emeritus professor in the Department of Genetics, Development and Cellular Biology have helped to usher in the genomics era to plant biology.
Their ground-breaking research on these genetic elements that can hop around the genome, reshape it and alter specific gene activity has spurred a new understanding about genomes and offered up some extremely valuable genomic tools.
Peterson and Robertson were contemporaries and in the case of Peterson, a colleague of Barbara McClintock whose discovery of the transposable elements Dissociator (Ds) and Activator (Ac) in maize in 1947 earned her the Nobel Prize in Physiology or Medicine in1983.
Transposons, or jumping genes are segments of either DNA—classical transposons or RNA-based retrotransposons that are highly mobile within and between chromosomes. They have captured the attention of geneticists in every sub-discipline from molecular to evolutionary genetics.
“Seventy-five percent of the corn genome is mobile,” says Peterson “and fifty percent of the human genome.” The rice genome is made up of twenty percent transposable elements. Regardless of percentages, transposons appear to be ubiquitous, found in eukaryotes and prokaryotes to varying degrees.
Transposons can cause mutations, silence some genes and activate others. They bring with them opportunities for genomes to respond to selective pressures by rapidly restructuring itself. But they can also cause lethal damage within depending upon where they jump. When jumping from a gene, transposons can often leave the abandoned gene non-functional.
These natural genetic engineering systems also serve as discovery tools, characteristics that can be exploited for gene mapping and targeted gene delivery.
Both Peterson and Robertson studied maize strains delivered to the California Institute of Technology in Pasadena, following exposure of maize seed to nuclear radiation from the experimental atomic blasts at Bikini and Anawetok atolls beginning in 1946.
Peterson's transposon system was discovered in one of these strains, while Robertson's was found in a strain of maize generously provided by Jerry Kermicle, professor emeritus of genetics at the University of Wisconsin, Madison, because it contained a class of mutants Robertson was studying at the time.
Peterson entered the scene with his discovery of the Enhancer-Inhibitor (En/I) mobile element in 1953. Robertson is best known for his discovery in 1978 of Robertson's mutator system now a well recognized and highly studied transposon family in maize that is widely used today as a tool for gene cloning.
Peterson later went on to advance the understanding that transposons are actual DNA insertions in genes that alter their expression by physical disruption, not simply regulatory or controlling segments as McClintock suggested.
Peterson, a native of Connecticut, received his undergraduate degree from Tufts University in 1947. In 1943 his academic education was put on hold by World War II. He served in the U.S. Navy and “it changed my whole perspective,” says Peterson. “I didn't think I would come back.”
When the war ended, Peterson felt lost when he returned to Tufts. In 1947, he spent a year at Cold Spring Harbor Laboratory in New York. It was here that he first met fellow Connecticut native McClintock and began a life-long collegial relationship.
Peterson completed his doctorate at the University of Illinois in 1953, and joined the University of California-Riverside. He joined the Iowa State faculty in the Department of Genetics in 1956 and later moved to the Department of Agronomy in 1960 where he still conducts rigorous experiments in corn genetics.
Robertson, born in Oakland, California, lived most of his young life in the San Joaquin and Santa Clara valleys. He too was affected by World War II. Following his service in the U.S. Army Air Force as a weather observer, Robertson completed his undergraduate degree at Stanford University and earned his doctorate at California Institute of Technology in 1951.
While at Stanford, Robertson discovered the field of genetics attending a genetics course taught by geneticist and Nobel Laureate George Beadle whom Robertson says “was the best teacher I ever encountered.”
Though he enjoyed genetics, studying fruit flies or other laboratory organisms at CalTech did not appeal to Robertson so he applied himself to plants, studying the Bikini Atoll corn mutants, laying the groundwork that eventually led to his discovery and characterization of Robertson's Mutator system.
McClintock's Ds and Ac elements responsible for variegation behaved as single genes but Robertson's Mutator system was the first of what is now understood to be a complex family of mutator elements in maize. They are elements that respond to a signal from the MuDR controlling element provoking a highly specific “cut and paste” style behavior when jumping around the genome.
Today a vast body of literature surrounding transposons is being added to daily and includes discoveries of new transposons, movement mechanisms, intricacies of site-specific recombination, and interactions with their host genomes.
“Many students and post-doctoral research fellows have chosen to study genetics because of their attraction to transposons,” says Peterson. Once gained, a clear understanding of transposon biology could offer tremendous opportunities for precise gene targeting efforts in generating transgenic plants.
