Shortly after Gilbert and Ptashne isolated the first repressors, Gilbert left the field of gene regulation and went on to develop a new technique for sequencing DNA, for which he received a Lasker Award in 1979 and a Nobel Prize in 1980.
Like Gilbert, Ptashne did not rest on his Harvard laurels. He continued to work on gene regulation and soon made his most notable discovery notable not only for its significance, but also for its elegance. To shorten a long story, Mark delineated the molecular basis of the lambda switch, which explains how the repressor and another regulatory protein called Cro interact with the DNA of the bacteriophage virus to switch genes on or off in response to two different environmental signals. Upon infecting a bacterial cell, the virus decides between two developmental pathways, called lysis and lysogeny. When the switch is ON, the virus multiplies exponentially. It commandeers the bacterial cell and kills it by lysis. When the switch is OFF, lysogeny occurs. The DNA of the virus inserts itself into the chromosome of the host bacteria and remains quiescent until the switch is reversed by signals from the environment.
Mark’s work explained beautifully how the lambda switch is constructed and how it is modulated by a positive and negative feedback system controlled by the two DNA-binding proteins, repressor and Cro, interacting with DNA and other components of the genetic machinery. Elucidating the lambda switch opened the field of transcriptional regulation as we know it today and provided the first general model to explain the switching between two developmental pathways that occur during the formation of embryos and cancers in higher organisms, including humans.
In 1985, Mark and his colleague at Harvard, Stephen Harrison, solved the crystal structure of the repressor bound to its DNA-binding address. This was the first atomic view of a transcription factor tickling its DNA. So over the 20-year period from 1965 to 1985, Ptashne pushed the lambda repressor from an abstract genetic concept to a purified protein molecule to an atomic structure. Not bad, even for a bold (and now bald) Harvard professor.
In more recent experiments, Mark used his insights from the lambda system to analyze gene regulation in higher organisms such as yeast. In typical Ptashne fashion, he formulated powerful new ideas that continue to influence all scientists working in the transcription field today. Let me briefly tell you one example of Mark’s influence. While studying the genes that allow metabolism of sugars in yeast (the so-called GAL4 system), he and his colleagues showed that transcription factors in higher organisms are constructed in modules. A typical transcription factor is made up of two discrete regions or domains; one domain directs the protein to the correct binding address on DNA, and a separate domain in the same molecule touches other proteins that activate the genetic machinery to turn on the gene. This discovery paved the way for other scientists to invent a powerful new technology—the so-called yeast two-hybrid system—that allows the identification of new proteins that interact with each other in the living cell. Using the yeast two-hybrid system, scientists have just recently discovered the master switch protein that directs the formation of the heart during development of the embryo.
The field of gene regulation, which began with Mark’s work on the lambda switch, deals with a subject that is central to all biology. As many as 15 to 20 percent of the 100,000 genes in the human genome encode transcription factors like the repressor and Cro. And not surprisingly, the transcription field is the most active field of basic research today, bar none. During the last 12 months, 11,000 papers on gene expression were published in the scientific literature—4,000 more than on cholesterol! The next chapter in the gene transcription story is now being written by biochemists who are reconstructing, in the test tube, the complex protein machinery of transcription. The ultimate aim is to show how multiple transcription factors and their various co-activator proteins interact to generate the serpintiginous networks that underlie normal cell development and function. Once this goal is achieved, it should be possible, one day, to learn how these networks are deranged in human disease, as in newborn babies with birth defects and in patients with cancer.
One final comment about Mark Ptashne’s boldness. Not only is Mark well known for his virtuoso performance in science, but he is also an accomplished violinist. According to legend, Mark decided to buy a Stradivarius violin in the late 1970s. Even though he had a good Harvard professor’s salary and had acquired some prize money, he still could not afford the Stradivarius. This was before his side ventures into biotechnology. So in typical Ptashne fashion, he approached the President of Harvard, Derek Bok, and attempted to convince him that Harvard should make him a loan. He argued that Harvard made special loans to faculty members so that their children could attend college. Ptashne had no children, and he argued that owning a Stradivarius would be the closest thing in his life to a child. I’ll leave it to Mark to tell us whether or not President Bok flipped the monetary switch ON or OFF!