Profile of Anthony R. Cashmore

Ask Anthony Cashmore about the unifying theme of his research career, which has spanned more than four decades on three continents, and he responds, “There was none.” Cashmore, a member of the National Academy of Sciences who recently retired as a professor of biology at the University of Pennsylvania, has spent much of his career studying how light controls gene expression in plants. However, his long journey through science is chronicled in quests as diverse as unraveling the structure of nucleic acids and understanding the biological basis of human behavior. Although Cashmore recently made waves for his provocative stance on the fiercely contested existence of free will, he is best known for identifying plant photoreceptor proteins called cryptochromes, which enable plants and animals to tell circadian time.

Anthony R. Cashmore.

Discovered by Cashmore in 1993 in Arabidopsis thaliana, a workhorse for plant molecular biology, cryptochromes mediate blue light signaling in plants, helping adjust growth and flowering to the amount and type of light in the plant's surroundings (1). Cashmore found that Arabidopsis cryptochromes have a carboxyl-terminal domain in their primary structure that distinguishes them from related DNA repair enzymes that help fix UV-ravaged DNA. Furthermore, Cashmore showed that the domain, when produced in Arabidopsis seedlings, mediated a constitutive light response, providing clues as to how cryptochromes act (2). Those early findings paved the way for later research showing that cryptochromes control circadian rhythms in people—a discovery that is part of a rapidly growing interest in sleep disorders.

Born in Auckland, New Zealand, Cashmore spent much of his youth in Manawaru, a pastoral village 100 miles south of Auckland where his parents owned a general store. Cashmore says, as a boy growing up surrounded by dairy farms, he had no inkling that he would one day pursue science in some of the world's preeminent research settings. A passing interest in biology triggered by an intriguing high school lecture on Mendelian genetics was among the few things that foreshadowed Cashmore's decades-long scientific career. The narrative of his early years does not feature childhood mentors or life-changing epiphanies, but it tells a story of hardscrabble self-discovery propelled by chance and scientific curiosity.

When Cashmore was 17 years old, he left high school for a job in Palmerston North, a campus town 90 miles north of New Zealand's capital. “More than any burning desire I may have had to be a scientist, it was really my father's fortuitous acquaintance with the head of the grasslands division of New Zealand's Department of Scientific and Industrial Research (DSIR) in Palmerston North that got me started,” Cashmore says. Cashmore then moved to the division's substation in Christchurch, where he enrolled at the University of Canterbury to study botany part time. “By then, I was quite determined to learn, and the job gave me a means to attend university part time,” Cashmore says. “My university education was essentially free, thanks to scholarships. My family did not have a lot of money to spare,” he adds.

In 1959, scholarship in hand, Cashmore moved to the University of Auckland, where he completed his undergraduate studies with a major in chemistry. Working toward his Master's degree in the laboratory of chemist Ralph Seelye, Cashmore analyzed the structure of organic compounds—called terpenes—derived from resins produced by plants native to New Zealand. Cashmore pursued his doctoral studies in the same laboratory and explored the structure of a terpene molecule called prostratin, derived from extracts of a Pimelea plant, that was known to curb cancer at low concentrations. Cashmore showed that prostratin is chemically similar to phorbol esters, which, through their effect on the signaling enzyme protein kinase C, promote cancer (3).

Cashmore recalls that Seelye allowed him to design and perform experiments with just enough guidance and autonomy. “I benefited greatly from the scientific freedom,” Cashmore says.

On the Shoulders of Giants

Scientific freedom is a theme that runs through Cashmore's scientific career, particularly during his postdoctoral training at the Medical Research Council Laboratory of Molecular Biology (LMB) in Cambridge, England, that, at one time, boasted a Nobel laureate on every floor and served as a stage for discoveries as fundamental as the double-helical structure of DNA.

Cashmore's circuitous path to the LMB began when, after earning his PhD in chemistry in 1966, he returned to DSIR, uncertain of the future. At a time when the term “molecular biology” had just entered the scientific vocabulary, Cashmore became interested in the structure of nucleic acids. He then met fellow DSIR scientist George Peterson, who had recently returned from a stint at Oxford University that entailed fashioning tools for DNA sequencing. Together, Cashmore and Peterson examined the suitability of hydrazine as a chemical tool for decoding DNA sequences, concluding that the reagent lacked the requisite specificity for use in sequencing (4). Ironically, molecular biologists Allan Maxam and Walter Gilbert later used hydrazine in their Nobel prize-winning DNA sequencing method memorialized in a 1977 PNAS paper (5). “A few years later, I mentioned my pessimism over chemical DNA sequencing methods to Frederick Sanger,” Cashmore says. In 1980, Sanger won his second Nobel prize for developing an alternative enzyme-based DNA sequencing method (6). With the help of Peterson's acquaintance, chemist Dan Brown, at the University of Cambridge, England, and a scholarship from DSIR, Cashmore obtained postdoctoral positions—first with Brown at the University of Cambridge and later at the LMB.

During Cashmore's time at the LMB, whose halls he shared with luminaries like Francis Crick, Sydney Brenner, and Frederick Sanger, Cashmore and molecular biologist John Smith applied a then novel approach to uncover the tertiary structure of a transfer RNA molecule, which helps ferry amino acids to the ribosomal protein factory in cells. Using genetic and chemical techniques to study the transfer RNA, Cashmore helped foretell the molecule's 3D structure long before its crystal structure was solved (7).

Cashmore's DSIR scholarship came with the condition that he return to New Zealand to conduct research there after his stint abroad. When he returned to DSIR's biochemistry division—after spending an intervening year at the University of California, Berkeley—his interest turned to plant molecular biology. He focused on a chloroplast enzyme, dubbed Rubisco, which touches off a series of photosynthetic reactions that plants use to turn atmospheric carbon dioxide into sugar. Cashmore found that the small subunit of Rubisco, thought to be Earth's most abundant protein, is made outside chloroplasts and in the cytoplasm of plant cells as an immature precursor unlike its large subunit and other chloroplast proteins, which are made inside chloroplasts (8).

Insights from Cashmore's studies on Rubisco went farther; because each plant cell harbors several thousand copies of the gene for the enzyme's large subunit, the reasoning went, the gene for the small subunit might similarly be found in many copies. Cashmore performed technically demanding biochemical studies on the small subunit's mRNA to discover that each genome harbored no more than five copies of the gene for Rubisco's small subunit, thus helping to piece together the puzzling story of the cytoplasmic synthesis of a chloroplast protein (9).

Despite the difficulty of performing molecular biological experiments with limited resources, Cashmore says, “I am quite proud of the work I did in New Zealand and consider it at least as impressive as anything I may have later achieved in the United States.”

Genes, Light, and Time

Eager to return to the United States for fresh challenges, Cashmore accepted a faculty position in 1979 as a visiting assistant professor at the Rockefeller University. After working with cell biologist Nam-Hi Chua for a year, Cashmore launched his own laboratory at Rockefeller with help from cell biologist Phillip Siekevitz. “I was among the earliest plant biologists to adopt cloning technologies, and our lab cloned many plant genes—from Arabidopsis, pea, tobacco, and tomato—whose expression was regulated by light. Naturally, I was interested in finding out what in the DNA sequence of these genes made them responsive to light,” Cashmore says. Thus began Cashmore's decades-long interest in light-controlled gene expression in plants.

Cashmore struck up a collaboration with molecular biologists Jozef Schell and Marc Van Montagu of the University of Ghent, Belgium, who had expertise creating transgenic plants. Together, the team found that the promoter sequence of the pea gene for Rubisco's small subunit conferred light-regulated expression when fused to a reporter gene and engineered into tobacco plants (10). Previous research had identified DNA sequences, called enhancers, in a simian virus dubbed SV40 that enhanced gene expression, regardless of their orientation and position relative to the protein-coding sequence of the regulated gene. “I heard Nobel Laureate Paul Berg talk at the Rockefeller, where he described his work on the SV40 enhancer sequence. I recall thinking the same mode of action might underlie regulated gene expression, including light regulation,” says Cashmore, who went on to show that the regulatory sequence of the gene for Rubisco's small subunit, like enhancer sequences, not only worked independently of orientation and position but also conferred light-controlled gene expression when fused to a reporter gene and expressed in tobacco plants (11). The finding held important implications for plant genetic engineering.

Of particular importance was Cashmore's related discovery of the mechanism by which some plant proteins made in the cytoplasm of plant cells are imported into chloroplasts. With his collaborators in Ghent, Cashmore found that a so-called transit peptide, a string of amino acids that serve as an address tag, in the precursor of the small subunit of pea Rubisco could help mediate the import of a reporter protein from the cytoplasm into the chloroplasts of transgenic tobacco plants (12). “The finding was quite striking at the time and was met with some surprise and skepticism at a meeting at Cold Spring Harbor,” Cashmore says. However, plant biotechnology giant Monsanto later used the finding to create transgenic plants resistant to the widely used herbicide RoundUp.

Cashmore's discoveries on gene expression in plants cemented his reputation in the scientific community and earned him a position as a director of the Plant Science Institute at the University of Pennsylvania in 1986. “I left the Rockefeller with some misgivings, because I had enjoyed living in New York and because work there had been very successful,” Cashmore says wistfully. “But my new colleagues at Penn more than compensated for any losses,” he adds.

Throughout his time at Penn, Cashmore pursued the saga of light-controlled gene expression in plants, revealing findings of fundamental importance along the way. Of note was Cashmore's discovery of proteins that bound to a conserved stretch of DNA, called the G-box, in the promoters of many light-controlled plant genes (13). “The G-box is related to the E-box in animal genes, where—as in plants—it regulates circadian genes, suggesting a common evolutionary history for both promoter elements,” Cashmore says. Extending those findings, Cashmore proposed that cells use the G-box, along with other DNA sequences, to vary gene expression through a regulatory interplay at the level of promoters (14).

Around this time, Arabidopsis was gaining popularity as a model for plant genetic studies. From a library of mutant transgenic Arabidopsis seeds developed by Kenneth Feldmann, a researcher at the technology firm DuPont, Cashmore's postdoctoral student Margaret Ahmad isolated a mutant Arabidopsis plant with reduced sensitivity to blue light. The mutant showed normal growth responses to red and far-red light, indicating that the plant's responses to phytochrome photoreceptors were normal. Cashmore entertained the alluring prospect that they may have identified a long-sought plant blue light photoreceptor, whose function was likely compromised in the mutant. Their thoughts were confirmed when they isolated the gene and sequenced it. The mutant gene seemed to be related to a blue light-activated enzyme called photolyase, which many organisms use to repair UV-damaged DNA (1). “Once I saw that the sequence of the Arabidopsis mutant gene was strikingly similar to that of photolyase, I knew we had hit the jackpot,” Cashmore says. Cashmore christened his newfound protein cryptochrome, a name coined earlier for photoreceptors thought to play a role in mediating blue light responses in mosses, ferns, fungi, and algae—a group of plants collectively called cryptogams.

Soon thereafter, other researchers found cryptochromes in fruit flies (15), where, as in plants (16), the proteins serve as photoreceptors that entrain circadian rhythms, helping plants and flies tell time. Researchers also found crytochromes in mammals, including people, where, in a fascinating evolutionary twist, cryptochromes serve not as a photoreceptor for entraining the circadian clock but as an essential component of the clock itself (17), a discovery with implications for the treatment of sleep disorders such as insomnia, sleep apnea, and narcolepsy.

Seasoned Scientist, Fledgling Philosopher

Cashmore's abiding interest in light-controlled gene expression has led to many discoveries. However, it was his 2010 PNAS Inaugural Article that catapulted him to the center of a fierce debate among scientists and nonscientists alike. Applying scientific rigor to a subject many scientists would be quick to dismiss, Cashmore's essay draws on supporting arguments from literature, philosophy, and neurobiology to refute the existence of free will, claiming that belief in free will is similar to belief in magic (18). Cashmore laments that, despite the near-total scientific consensus that genes, environment, and chance shape most human traits, many biologists believe that most people can exercise free will to control their actions. Because an individual's actions have neurobiological underpinnings, the feeling of mastering our actions by exercising free will, Cashmore argues, is an illusion. “It puzzles me enormously that most biologists appear to believe in free will. The driving force behind this illogical belief must be the minute-by-minute reminder we all experience of the correlation between conscious thought and behavior,” Cashmore says.

Cashmore's arguments, which bear unsettling implications for our notions of credit and blame and for the criminal justice system, evoke a bedeviling mystery. Cashmore says the mystery of the chicken and egg link between conscious and unconscious brain activity has piqued his interest for decades. “Some people would say it has become an obsession,” he adds. “I would always find an excuse to sneak a discussion of free will into lectures of biochemistry, genetics, or molecular biology.”

Conceived as a popular book on free will that is yet to materialize, Cashmore's Inaugural Article drew more criticism than praise. Critics reviled Cashmore's viewpoint as misguided. Undaunted, Cashmore counters, “Unless someone comes up with a molecular mechanism for free will, the concept should be discarded.” Part of the criticism of Cashmore's writing stemmed from his likening of belief in free will to belief in religion, both of which, he argues, amount to belief in fantasy. However, he explains, the illusory sense of responsibility that belief in free will might confer on people could provide an evolutionary advantage, partly explaining its endurance.

Through contributions ranging into chemistry, genetics, molecular biology, and philosophy over more than four decades, Cashmore cut a wide swath of scientific inquiry. His discoveries have propelled basic research and borne practical benefits. However, this protean researcher, who retired from the University of Pennsylvania this July, says he is first and foremost a chemist with a strong interest in biology. “People are nothing but a complex mixture of chemicals, and many might take exception to this viewpoint. But the enormous complexity and combinatorial nature of this mixture, constantly tweaked by the environment, is awe-inspiring, which is why the viewpoint should not be considered demeaning in any way,” Cashmore says.