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Understanding
Gene Functions Through Mutation
By Karen Kreeger
For Nancy Craig ’73,
one of the most important of Bryn Mawr was the
support she had as a woman in science. "There
was no one asking in an unbelieving way, 'Oh,
you're a chemistry major?’" she says.
"And goodness knows I've encountered that. There
is still an element of incredulity about women
doing science." But at Bryn Mawr this simply was
not an issue, "and that's why I found it such
a powerful place," she says.
Craig recalls a high-school
teacher of English, German and Latin who was instrumental
in helping her decide where to go to college.
She believed California was too provincial and
that Craig should look outside of the state. "She's
the one who told me about places like Bryn Mawr
and advocated that I should look more broadly,"
says Craig. "I was never even east of the Rockies
until I flew over them to come out to school."
After graduating from Bryn
Mawr, Craig had to decide whether to go on to
medical school or graduate school. She realized
that she was more interested in how and why people
got sick than in treating disease. Craig went
on to graduate school at Cornell University studying
how bacterial cells respond to DNA damage and
how viruses grow in those cells. She earned her
Ph.D. in biochemistry in 1980.
Jumping
Genes
Craig accepted a postdoctoral
fellowship at the National Institute of Mental
Health and in 1984 joined the Department of Microbiology
and Immunology at the University of California,
San Francisco, as an assistant professor. She
started working on a type of mobile DNA called
transposons. These are pieces of DNA that move
to different locations within a cell and sometimes
between cells. Transposons — so-called jumping
genes — were discovered in the late 1940s
by Nobel laureate Barbara McClintock.
Craig has been at Johns Hopkins
University School of Medicine in the Department
of Molecular Biology and Genetics since 1992.
She was also awarded an investigatorship with
the Howard Hughes Medical Institute in 1992, which
she still holds.
Currently Craig runs a lab
that includes about 10 graduate students, postdocs
and technicians. "I've heard people say that science
is a lonely endeavor," she notes. "It's almost
archetypal. But in fact it's a very interactive
job. I could characterize my job as running a
small business. I have to teach, coordinate and
facilitate, and all of that is very people-oriented."
For Craig, the most gratifying
part of her job is understanding how nature works.
"I don't work at the bench anymore, but I'm still
grabbed by these questions," she says.
Transposable
Elements
Craig’s basic research
on transposons and how, at the molecular level,
they make the jump within and between cells has
far-reaching implications. Although she doesn’t
work directly with HIV, Craig’s studies of
transposons increase our understanding of how
HIV replicates and moves from cell to cell. Retroviruses
like HIV are actually transposable elements —
copies of them go into new cells, where they join
DNA and become integrated into human chromosomes.
Transposable elements are
also useful as research tools to ascertain the
functions of genes. Craig works with a bacterial
transposon called Tn7. "We've been able to make
many types of Tn7s and put a molecular tag in
the middle of the element, such as genes for antibiotic
resistance, to follow the transposon when it moves,"
she explains.
Usually Tn7 is very selective
about where it inserts and in which types of DNA
it will insert, but Craig's lab has made a version
of Tn7 that is not so picky — that is, it
can be inserted into many different places in
a genome. Craig and Jeff Boeke, a yeast biologist
at Johns Hopkins, have made tens of thousands
of yeast mutants using Tn7. With this technique,
they were able to associate the changed behavior
of a particular mutant with a Tn7 insertion at
a particular site, allowing easy identification
of the mutated gene. Knowing how a mutated gene’s
behavior is altered often provides important insights
into the function of that particular gene when
it is not mutated.
"We're interested in applying
this technique to bacteria to study what kinds
of genes are important," Craig says. For example,
E. coli, the common human gastrointestinal-tract
bacteria, has about 4,300 genes. E. coli
has been studied intensively for 50 years, yet
the functions of 30 percent of its genes remain
a mystery. "Sorting out who does what is important,
and to do that we can use insertion mutants made
with Tn7," she says.
Another distinctive feature
of Tn7 is that it can preferentially hop onto
bacterial plasmids that move between cells. This
phenomenon underlies the dissemination of antibiotic
resistance genes contained within the transposons.
"Imagine you have an awful infection in the gut
because of a pathogenic strain of E. coli,
and you're prescribed tetracycline," Craig explains.
"There might be other bacteria around that carry
transposons having tetracycline-resistance genes
on them. If those antibiotic resistance genes
can move into your pathogenic E. coli by
hopping onto a plasmid, then those E. coli
will also become resistant to tetracycline. So
it's the movement of transposons between cells
that underlies this terrible problem of antibiotic
resistance." Tn7 itself carries resistance to
one of the drugs that is often prescribed for
urinary tract infections — trimethoprim,
which is one of the drugs in Bactrim.
Career
Advice
Craig has been greatly gratified
by her career thus far, and advises up-and-coming
scientists to "do what you really want. I see
far too many students today who ask, 'What area
of research would be a good career choice for
me, in terms of what's "hot"?' I think, especially
in science, you simply have to be fundamentally
motivated about a problem because it fascinates
you."
Craig says that Bryn Mawr
provided her with a wonderful foundation. Part
of that is the excellent education she received.
But another important part is "this very powerful
sense of self as a person, and I guess that's
particularly an issue for going into such a male-dominated
field," she concludes.
About the Author
Karen Young Kreeger is a science
journalist who writes on biomedical and women’s
health topics, as well as careers in science.
Her most recent work has appeared in Bioscience,
Genome Technology, Muse and The
Scientist.
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