PROFILE Profile of Nancy L. Craig ancy Craig studies how DNA again. This triggers the transformation moves from place to place: from a quiet hitchhiker in the bacterial Na deceptively simple quest that genome to an active virus bent on repli- has revealed how transposons, cating itself and destroying the bacterial or so-called “DNA cut-and-paste ele- cell (1). ments,” snip themselves from one loca- tion on the chromosome and resettle in From Virus to Transposon another. Craig, a professor of molecular Craig’s love of cutting and pasting led her biology and genetics at Johns Hopkins to the National Institutes of Health University School of Medicine (Baltimore, (NIH) for a postdoctoral position in the MD), an investigator with the Howard laboratory of Howard Nash, inventor of Hughes Medical Institute, and a recently the first in vitro system for studying site- elected member of the National Academy specific recombination, such as the in- of Sciences, has helped uncover how tegration and excision cycle of phage transposable elements jump from one po- lambda. Under his guidance, Craig hoped sition to another to cause disease and to determine molecules necessary to re- sculpt the seemingly infinite variety of locate the virus within the genome. genomes on Earth. While she was at the NIH, a 1982 Even before scientists revealed in 2001 Nature article (2) piqued her interest. that half of the human genome was packed “It was about the seventh transposable with transposable elements, it was clear element described so far, aptly named from Barbara McClintock’s classic studies Tn7, that inserted into a single specific in the 1940s that these so-called “jumping site. That was very unusual because all genes” alter genetic information. The the transposable elements that we knew nomadic DNA segments, which are pres- about at that time went into many ent in bacteria, corn, fruit flies, and most Nancy L. Craig. different sites. So, how did this element other organisms, can profoundly modify choose its target?” Nash, whom Craig the features of the genome. Craig’s work thinking, this is amazing! I was certainly describes as “an exceptional scientist and has revealed the complex mechanisms interested in biology before, you know, very good mentor,” had also seen the ar- that allow these elements to move, as well but that was the moment I chose to ticle, anticipated her interest, and brought as the tricks a virus uses to incorporate study DNA.” it to her attention. Although she already into, and escape from, the genome when had plans to continue her research on times get tough. Craig has shown that Cutting and Pasting lambda phage as an assistant professor at transposons move by common mecha- Craig enrolled in a doctoral program at the University of California, San Fran- nisms, yet they each have their own Cornell University in 1973. Her thesis cisco (UCSF), the Nature paper shifted character. The hyperactive mutant trans- project explored the chemistry required to her career path. Craig became fascinated posons developed by Craig’s team have repair DNA. Virtually all cells have cel- by the specificity of Tn7 and sought NIH become frontline tools for genetic lular systems to repair damaged DNA; the funding to explore the proteins re- engineering. process is vital to life. What really in- sponsible for breaking and joining DNA trigued Craig about this system, however, during translocation. Get Out of California was exactly how the bacteria-infecting When she was a young girl, Craig’s parents virus known as bacteriophage lambda was Recipe for Transposition enthusiastically supported her interests, able to integrate into the chromosome and Craig wanted to reconstruct the entire supplying her with the requisite chemistry then snip itself out when DNA damage transposition event in a test tube, honing set that every budding scientist needs. endangered the bacterium. Lambda, like in on the specific DNA sequences targeted But Craig says she encountered few fe- other viruses, can integrate into genomes by Tn7. Although the research lent itself male scientist role models during her at specific sites and proliferate for many to tantalizing clinical applications, such early years. Eventually a high school generations. However, when the bacterial as gene therapy, Craig focused on the teacher named Caroline Ross put Craig DNA is damaged, the virus removes transposon’s basic mechanism. Tn7 is on the fast track to a career in science. itself from the genome and makes escape a highly site-specific transposon that “You can’t just stay in California,” Craig modules. This aptly named “SOS re- hones in and predictably inserts itself into recalls her teacher saying. Inspired to sponse” enables the virus to leave the cell a particular DNA sequence. However, break into the male-dominated field of so it can infect another bacterium and when that target sequence is absent from science, Craig enrolled at Bryn Mawr integrate into its genome. A molecule the chromosome, Tn7 will seek out an- College in Pennsylvania. “It was empow- called the repressor is responsible for other. Craig was determined to reveal the ering to go to a woman’s college, where keeping the phage, or virus, quiet while it secrets of the transposon’s flexibility. Tn7 they studied physics and math and it is in the bacterial genome. The repressor requires an entourage of proteins to per- wasn’t an odd thing,” says Craig. “When silences the phage genes required to form its integration and removal from I was growing up, that was an issue.” manufacture viral particles, keeping only sites in the genome, and Craig’s goal was While completing a major in biology the repressor gene active. to identify the main players. She made and chemistry, Craig found her calling. Craig discovered that a cellular protein “During my sophomore year, I had three called RecA interacts with the ssDNA classes in one day—biology, physics, and resulting from damage and helps destroy This is a Profile of a recently elected member of the Na- chemistry, and all the professors hap- the repressor. Without the repressor, the tional Academy of Sciences to accompany the member’s pened to talk about DNA. I remember rest of the viral genes are switched on Inaugural Article on page 21966 in issue 51 of volume 107. www.pnas.org/cgi/doi/10.1073/pnas.1212357109 PNAS | September 4, 2012 | vol. 109 | no. 36 | 14283–14284 Downloaded by guest on September 25, 2021 cell fractions containing the four trans- recombination system. The immune mammalian cut-and-paste element position proteins that are encoded by the system generates diverse antibodies to known,” she says. As it turns out, the transposon, mixed them in the test tube fight bacteria, viruses, and parasites by piggyBac transposon is particularly with transposon DNA, and found that essentially cutting and pasting gene seg- efficient at inserting into foreign ge- breakage and joining occurred. “We ments together to generate a diverse nomes. In 2008, Craig discovered what could finally do the Tn7 transposition collection of antibody genes during V(D)J might be the key to the element’ssuc- reaction in the test tube!” she recalls. A recombination. The RAG protein cata- cess: piggyBac appears to bypass paper published in Cell in 1991 describes lyzes this gene shuffling by promoting an important DNA synthesis step. how the breaks and joins occur (3). DNA breakage and joining events. A Transposons usually require a little as- Craig wanted to know the identity of all coveted hypothesis had been that trans- sistance from their host repair proteins, the proteins involved in Tn7 transposition posons gave rise to this very organized DNA polymerases, after they have re- and continued their purification from V(D)J system. Craig showed that the hAT located to regenerate intact duplex cell extracts. After 2 years, Craig cracked and RAG DNA breakage mechanisms are DNA. In a 2008 paper (7), Craig re- the ingredients for transposition: the related. However, since working on vealed that piggyBac repairs DNA using transposon, the DNA containing the tar- another family of transposons called a little extension on the ends of the get site, and the four proteins essential Transib, including one named Hztransib transposon that attaches directly to the for Tn7 transposition in vivo. She pub- that was isolated from the cotton and corn host. “Itmightbepartofareason lished her findings in Cell in 1993 (4). bollworm pest Helicoverpa zea, Craig has that [the transposon] moves efficiently: Together, the two papers laid a founda- discovered the first active transposon of It doesn’t have to depend upon host tion for the rest of Craig’s career. “We the Transib family that can undergo functions to move.” Craig believes she now had a system that we could study excision and integration. Craig’s latest in canharnessthewell-knownefficiency in the test tube so that we could dissect vitro experiments have shown that this of piggyBac to create a power tool the molecular details.” After 8 years at new transposon uses the same mechanism for genome engineering. UCSF, Craig moved with her partner as does RAG, suggesting that Transibs The demand for a hyperactive trans- to Baltimore, where she accepted a posi- are more closely related to RAG than poson, however, is arguably less than tion at Johns Hopkins University and to the hAT proteins. the demand for building sedate elements began to work on other transposons in that hone in on specific, safe places. addition to Tn7. Crystal Structure of Hermes Researchers have not yet perfected the In 2005 (6), Craig got a high-resolution ability to direct a transposon to a single, Many Transposons, One Mechanism glimpse of the eukaryotic transposase specific site, and clinical trials are Craig expanded her work to include Hermes.