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UC Berkeley Press Release

Memorial for late yeast expert Robert Mortimer

– A memorial service is scheduled for Sunday, Oct. 28, to honor Robert K. Mortimer, an eminent yeast expert and professor emeritus of molecular and cell biology at the University of California, Berkeley, who died Aug. 10 from complications of Alzheimer's and Parkinson's diseases. He was 79.

The service will be held at 2:30 p.m. at the Northbrae Community Church, 941 The Alameda, in north Berkeley.

Robert Mortimer
Robert Mortimer

Mortimer was known worldwide for his role in making yeast one of the premier laboratory organisms in which to study the operations of genes and chromosomes. At a time when these one-celled fungi were much harder to study than the more popular fruit flies or corn or multi-celled fungi, he developed techniques that made the study of yeast DNA easy and paved the way for decades of fundamental genetic research. As he championed yeast for genetic studies, he also created a library of thousands of separate strains of Saccharomyces cerevisiae - the common brewer's or baker's yeast - that he freely offered to colleagues.

"Any investigator could browse the catalog and request any strain and receive it promptly for nothing more than the cost of a stamp," said Jasper Rine, UC Berkeley professor of molecular and cell biology. "This generous service did much to create a culture of sharing, which still blesses the yeast genetic community to this day."

The Genetics Society of America recognized Mortimer in 2002 with the George W. Beadle Award for his many contributions to the creation of a community of approximately 10,000 researchers who strive to uncover more of the secrets of Saccharomyces.

Upon his retirement in 1991, Mortimer turned his attention to the strains of yeast used in making wine and came to believe, as do many European winemakers, that fermentation by the naturally occurring, indigenous yeast on grapes produces a more complex wine. He met with many French, Italian and California winemakers as he studied the "wild" yeasts in vineyards and showed winemakers evidence that different strains of yeast work sequentially during fermentation to produce complexity. His work contributed to a growing American interest in natural yeast fermentation instead of the use of industrial yeast.

"He gave legitimacy to the whole concept of using indigenous yeast," said Bill Dyer, a consulting winemaker and owner of Napa Valley's Dyer Vineyard who for years was winemaker at Sterling Vineyards in Napa Valley and met Mortimer in the early 1990s when Dyer was experimenting with natural yeast fermentation. "Here was a researcher, a scientist obviously of some repute to be on the faculty at Berkeley, and he was paying attention to what we were doing and didn't think it was crazy."

In a further expansion of his later research interests, Mortimer was part of a team that in 2003 announced evidence that S. cerevisiae was used to ferment wine at least as early as 3150 B.C. in Egypt. The leader of that study, Duccio Cavalieri, now a professor of microbiology at the University of Florence, fondly recalled in an e-mail how Mortimer taught him to dissect yeast and then told him, "and now you can challenge (Gregor) Mendel," the Austrian "father of genetics" know for his experiments on inheritance of traits in peas.

"This was typical of Bob (to) challenge an established concept and not believe, exposing nature to the light of science with a well-planned experiment," Cavalieri wrote. "I really consider Bob as 'il maestro' in the sense that the renaissance artists called their teachers - someone that could teach me through the understanding of my inner emotions and help me in taking the right avenues in my life and my career."

Born in Didsbury, Alberta, Canada, on Nov. 1, 1927, Mortimer graduated in 1949 from the University of Alberta with a magna cum laude in physics and, rejecting a career in oil and gas exploration that was ramping up in Alberta, enrolled as a graduate student in UC Berkeley's biophysics department. Working with Cornelius Tobias, the late UC Berkeley professor of medical physics, he explored the mechanisms used by yeast to repair radiation and chemical damage to yeast's chromosomes.

"The so-called RAD genes that were discovered in these studies are the foundation of most contemporary studies of how higher cells repair damage to their DNA," Rine said.

Mortimer obtained his Ph.D. in biophysics from UC Berkeley in 1953, served as an instructor for three years, and then joined the faculty as an assistant professor of genetics in 1956. He became a full professor in 1966, chaired the division of medical physics in 1972-1978, and served as chair of the Department of Biophysics and Medical Physics from 1984 to 1987. He also was affiliated with Lawrence Berkeley National Laboratory, where he served for a year as acting director of the lab's Human Genome Project.

According to Rine, Mortimer realized early in his career that a genetic map of yeast would be required to figure out just what X-rays did to chromosomes that led to cell death, and he set out to create such a genetic map. This was no easy task because Saccharomyces yeast have 16 chromosomes, all much smaller than those of other lab organisms. Nevertheless, by the mid-1970s, Mortimer had a workable map with multiple mutations marking all the chromosomes.

Even with the map, however, yeast genetics would not have taken off had Mortimer not also found a way to dissolve yeast's tough spore casing, or ascus, so that the chromosomes could be studied. Reasoning that "all forms of life are eaten by something, and hence some creature must be able to digest Saccharomyces asci," he and colleagues discovered in 1959 that "if they extracted the juice from the guts of snails, that juice could be used to digest the asci, liberating the spores for genetic analysis," Rine said.

"This insight threw open the power of genetic studies using yeast," Rine added. "Rather than analyzing a few meioses, it was now possible for one person to analyze thousands of meioses, which Mortimer, his students and post-doctoral fellows have done." Meiosis involves two successive cell divisions leading to gametes - the equivalent of sperm and egg - with only half the normal complement of chromosomes.

This approach led Mortimer to focus on gene conversion, a process that allows the information on one chromosome to be essentially copied into the homologous chromosome, replacing the information that was there. The work of Mortimer and colleagues, including UC Berkeley geneticist Seymour Fogel, provided essential insights into how genetic recombination, the foundation of all sexual reproduction, operates, and led to methods now used for precise gene replacements in many different organisms.

In addition to developing the field of yeast genetics, Mortimer set up a Saccharomyces stock center housing many thousands of strains of yeast that his group created as well as strains obtained from other scientists. In his later years, Mortimer also developed new strains of yeast with improved traits for wine fermentation.

According to Alex Nichols, professor emeritus of molecular and cell biology, Mortimer was known for his exceedingly well-organized lectures that gave "a sense of historical development rich in background," he said. "He was a mild-mannered, contemplative teacher that students warmed up to - he was always available and understanding."

After his retirement, he became a visiting scholar at the University of Florence in Italy and collaborated for nearly a decade with researchers there and at the University of Bologna to produce eight papers on the varieties of yeast inhabiting Italian and California grapes and those involved in the natural fermentation process.

"Bob Mortimer was one of the campus's kindest, most generous members who is justly credited as being the father of his field," said Rine. "Bob's pioneering work led to thousands of scientific careers dedicated to the study of that glorious microbe, the yeast Saccharomyces cerevisiae, responsible for such pleasures of life as beer, bread, wine."

Mortimer's interest in single-celled fungi extended to the edible, multi-celled kind, and mushroom hunting was one of his favorite hobbies, as were fly fishing, hiking and gardening.

Mortimer is survived by his wife Mary, of Berkeley; sons, Douglas of Berkeley and Bruce of Paradise, Calif.; daughter, Barbara Mortimer of Redwood City, Calif.; and three grandchildren. He and his wife had a son, Donald, who died in 1993.