Chromatin Structure and the Nucleosome: the Work of Kensal E. van Holde

Abstract

The Effect of Trypsin on Nuclease-resistant Chromatin Fragments

(Sahasrabuddhe, C. G., and Van Holde, K. E. (1974) J. Biol. Chem. 249, 152–156)

Kensal Edward van Holde was born in 1928 in Eau Claire, Wisconsin. At an early age, he became interested in organic chemistry. However, this interest dissolved when he enrolled at the University of Wisconsin and took his first organic chemistry course. He recalled, “I found learning multitudes of types of reactions, without a hint as to meaning, mechanism, or purpose, infinitely boring. Worse, I proved incompetent in the laboratory. My product yields were miserable, my melting points embarrassing” (1).

Kensal E. van Holde

Despite this experience, he stuck with chemistry and eventually became enamored with physical chemistry. During his senior year, van Holde carried out a research project in J. W. Williams' laboratory, using old Svedberg analytical ultracentrifuges. His time there cemented his love for research and led to his decision to remain in Williams' laboratory for graduate school. van Holde's thesis project centered on synthetic polymers, and he graduated in 1952.

Convinced that there was “nothing to study in the biochemical world except a lot of meaningless reactions and ill-defined substances” (1), van Holde became a polymer chemist at E. I. DuPont de Nemours after graduating. However, after 3 years, he realized he was not suited for industrial research and wrote to Williams asking for help. Williams offered van Holde a postdoctoral position, which he gladly accepted.

Back in Wisconsin, van Holde returned to polymer work and began to study the creep of nitrocellulose. In his spare time, he tried to figure out how to shorten the time it took to do a sedimentation equilibrium experiment (a method he made use of in earlier polymer work). Collaborating with Robert Baldwin, van Holde came up with the idea of shortening the solution column height and reduced the experiment time from 1 week to less than a day. The resulting publication (2) was widely cited, and “short column sedimentation equilibrium” became a popular experimental method.

In 1957, van Holde joined the faculty at the University of Illinois at Urbana-Champaign. There his research centered on the ultracentrifuge and other physical techniques such as light scattering and circular dichroism. However, at the same time, he developed an interest in biochemistry, a subject he couldn't easily explore in the physical chemistry department he had joined at Illinois. So, in 1967, van Holde left his tenured position and became a professor in the department of biochemistry and biophysics at Oregon State University.

With a fresh outlook and access to biochemically oriented students and postdoctoral fellows, van Holde embarked on a new research program. He became interested in electric dichroism, a technique whereby macromolecules are oriented in an intense electric field and the absorption of light polarized in a parallel or perpendicular field is used to evaluate internal macromolecular structure in solution. van Holde and his graduate student, Fritz Allen, built a new and sensitive instrument to perform this technique and tested it on tobacco mosaic virus and DNA. The instrument worked beautifully on both macromolecules, and van Holde began to look for something new on which to try his technique. He chose chromatin, in part because Irvin Isenberg, a colleague in the department, had developed a strong interest in histones and chromatin.

At that time, the favored model for chromatin structure was one in which histone-coated DNA followed a gentle, more or less uniform supercoil (3). The model predicted that the dichroism would have a large positive value. However, the dichroism van Holde and his postdoc Randolph Rill observed was negative. Attempting to make sense of these observations, van Holde recalled a paper by Clark and Felsenfeld (4) that indicated that part of the DNA in chromatin was much more accessible to nuclease digestion than the remainder, suggesting that chromatin might contain stretches of uncomplexed DNA in addition to those that were protein-bound. So, he had Rill carry out limited digestions of chromatin using micrococcal nuclease. After the digestion, Rill was left with residual particles that had a compact structure and contained both DNA and histones (5). The DNA in the particles had a circular dichroism spectrum unlike that of B-DNA or whole chromatin, suggesting it had adopted an unusual conformation.

van Holde's graduate student, Chintamin Sahasrabuddhe, was able to further improve the residual particle preparation and set about its characterization, as detailed in the Journal of Biological Chemistry (JBC) Classic reprinted here. They discovered that the particles were spherical and contained at least half the DNA in chromatin. Each particle carried one piece of DNA about 110 bp long and also contained about 100,000 Da of protein. Furthermore, they found that the protein was essential for the integrity of the particles.

The particles van Holde and his colleagues helped discover eventually came to be known as nucleosomes, and his JBC publication kicked off a race to determine their fine structure and functional significance in chromatin. Eventually, results from van Holde and other laboratories provided a new model for chromatin in which the DNA was wound around a core consisting of an octamer of histones.

van Holde remained at Oregon State University for the rest of his career, eventually retiring in 1993. From 1977 to 1993, he was the holder of an American Cancer Society Research Professorship. He also taught the physiology course at the Marine Biological Laboratory from 1962 to 1967 and directed the course from 1977 to 1980.

van Holde has received numerous honors for his contributions to science, including election to the National Academy of Sciences and the American Academy of Arts and Sciences. Currently he is an Associate Editor for the Journal of Biological Chemistry.¹