In the 1950s, near the beginning of Gall’s career, scientists were discovering the roles of DNA, RNA, and protein through studies of prokaryotes, single cell organisms that have no nuclei. At the same time, they were grappling with basic questions about how the functions of these molecules related to structures in multicellular organisms, such as humans and animals, which sequester their DNA in nuclei (eukaryotes). Few tools and techniques existed for probing these issues.
Gall was well aware of these limitations and repeatedly blazed new paths to solutions of longstanding biological conundrums. His knack for choosing the appropriate organism for studying a particular problem began during his PhD, when he focused on the structure of chromosomes in amphibian eggs, or oocytes. Loops extend from the axes of these exceptionally large chromosomes, creating a bristly appearance that prompted early cell biologists to name them “lampbrush” chromosomes. Their size permits observations and manipulations that are difficult or impossible with smaller chromosomes. In the early 1960s, by which time Gall was running his own lab at the University of Minnesota, scientists knew that genes were made of DNA and resided in chromosomes, but no one knew how many DNA molecules composed a single chromosome or what held the genes together. By treating the lampbrush chromosomes with an enzyme that cuts DNA and measuring the speed at which the DNA broke, Gall gathered strong evidence to suggest that each chromosome consists of a single DNA double helix. He also showed that the loops of the lampbrush chromosomes consist of genes that are being copied into the RNAs that the egg stockpiles for use as it develops into a new individual.
Gall also used amphibian oocyte nuclei to study the envelope that encases the nuclear contents. Using an electron microscope, he saw pores in this membranous pouch and demonstrated that these nuclear pores have eight-fold symmetry. He thus provided the first physical characterization of the structures that we now know control the traffic of crucial molecules into and out of the nucleus.
In 1964, Gall moved to Yale. There he studied the RNA of ribosomes — the cell’s protein-manufacturing factories. He discovered that, during egg formation, the genes encoding this RNA are duplicated multiple times in the nucleus, but outside of the chromosome. Gall thus unveiled the first example of gene amplification, a strategy by which some types of cells — such as those destined to create tumors — generate large quantities of particular DNA sequences at specified times or under certain circumstances. Furthermore, he established that nuclear genes in eukaryotes can dwell outside of the chromosome. Similar findings were reported independently by Oscar Miller and by Donald Brown and Igor Dawid.
These observations set the stage for the development of in situ hybridization, a powerful technique that allows scientists to locate specific RNA or DNA sequences in particular regions of the cell. For years, Gall had wanted to find a way to detect individual genes within chromosomes, and he realized that he now could begin devising such a technique. The amplified ribosomal RNA genes were the key, because they provided such a large target inside the nuclei of the developing oocytes. He and Mary Lou Pardue, a graduate student in his lab, squashed cells from the ovary onto a microscope slide. Then they generated a radioactive version of the ribosomal RNA, which they spread on the slide, hoping that it would adhere to the corresponding DNA sequences. They washed away the RNA that didn’t stick and placed the slide on X-ray film. The radioactive RNA exposed the film precisely where the amplified ribosomal RNA genes lay in the nuclei.
In situ hybridization quickly became one of the most widely used techniques in cell biology. It is still the standard method for mapping genes within tissues, nuclei, or chromosomes. It has proved to be an indispensable tool for pinpointing when and where particular genes turn on and off in the developing embryo, information that can hint at their physiological roles. In the 37 years since Pardue and Gall published their first paper on in situ hybridization, scientists have refined the technique. They now use different colored fluorescent molecules to adhere to multiple sequences within a single cell, thus generating an exquisitely detailed picture of genes and gene activity.
Gall then employed in situ hybridization to locate so-called satellite DNA on the mouse chromosome. He and Pardue found that this DNA, composed of repeated short sequences, lay in a particular spot that was known to lack genes. This was the first demonstration that highly repeated sequences reside at specific regions of the chromosome and it provided an explanation for the absence of genes in that region.
Gall went on to demonstrate that the protozoan Tetrahymena thermophila generates many copies of free ribosomal DNA molecules — and another example of DNA amplification independent of chromosome duplication. He and Elizabeth Blackburn used these DNA molecules to study chromosome ends, a line of inquiry that led to the discovery of telomerase (see description of the 2006 Albert Lasker Award for Basic Medical Research).
After more than five decades in the lab, most investigators would have long ago left the hands-on research to their students and postdoctoral fellows, but Gall is still at the bench. He is currently studying a structure in the nucleus, the Cajal body, which was described in 1903, but whose function is still not clear. His results suggest that Cajal bodies, which are present in all eukaryotic organisms, are assembly sites for the machinery that processes messenger RNAs, the protein templates. Although the Cajal body has been neglected for most of the century since its first identification, these new insights, pioneered by Gall, have stimulated much recent excitement in the field of nuclear structure and function.
In addition to exploring the nucleus, Gall has distinguished himself as a superb role model and mentor. Through respect, support, and the high standards that he sets in his research, he has nurtured a large number of young investigators who have gone on to achieve great success as independent researchers and leaders. In particular, he has built a strong record of training female scientists, three of whom — Mary Lou Pardue, Susan Gerbi, and Elizabeth Blackburn — served as American Society for Cell Biology presidents. Gall never made a conscious decision to promote women in science; rather, he realized before many of his peers the wisdom of accepting good students into his lab, regardless of gender.
Gall’s studies on diverse problems in cell biology in many different organisms have revealed fundamental properties of chromosomes and the nucleus. He developed one of the most important techniques in cell biology. His work spans more than half a century and reflects his keen mind, focused efforts, experimental gifts, and the power of teaching by example. Gall’s legacy has already permeated cell biology textbooks and will reach far into the future through the biological problems and people he has touched.
by Evelyn Strauss
Key publications of Joseph Gall
Gall, J.G. (1963). Kinetics of deoxyribonuclease action on chromosomes. Nature. 198, 36–38.
Gall, J.G. (1968). Differential synthesis of the genes for ribosomal RNA during amphibian oogenesis. Proc. Natl. Acad. Sci. USA. 60, 553–560.
Gall, J.G. and Pardue, M.L. (1969). Formation and detection of RNA-DNA hybrid molecules in cytological preparations. Proc. Natl. Acad. Sci. USA. 63, 378–383.
Pardue, M.L. and Gall, J.G. (1970). Chromosomal localization of mouse satellite DNA. Science. 168, 1356–1358.
Blackburn, E.H. and Gall, J.G. (1978). A tandemly repeated sequence at the termini of the extrachromosomal ribosomal RNA genes in Tetrahymena. J. Mol. Biol. 120, 33–53.
Gall, J.G., Bellini, M., Wu, Z., and Murphy, C. (1999). Assembly of the nuclear transcription and processing machinery: Cajal bodies (coiled bodies) and transcriptosomes. Mol. Biol. Cell. 10, 4385–4402.