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Bruce Voyles
Patricia Armstrong Johnson Professor of Biological
Chemistry; and Professor of Biology; Senior Faculty Status
Ph.D., 1974, Purdue University; Research Associate, 1974-1977, Michigan
Cancer Foundation; Virology and Microbiology.
Tel: 641-269-3038; FAX: 641-269-4285;
Office: Science-1103; Email: voyles@grinnell.edu
- Biology 150: Introduction to Biological Inquiry: Emerging and Re-emerging Pathogens [Bio 150 Schedule - .PDF]
- Biology 235: Molecular Genetics
- Biology 251: Molecules, Cells, and Organisms
- Biology 365: Biology of Prokaryotes
- Biology 385: Virology
I am investigating genes that may play a role in allowing the soil bacterium
Acinetobacter baylyi strain ADP1 to survive under starvation conditions. Many
bacteria spend most of their time in environments with limited nutrients and have
evolved a variety of mechanisms for survival under such conditions. I am interested
in identifying the genes involved in these mechanisms. To this end, my students and I
used a genetic strategy to identify genes induced during stationary phase
Acinetobacter. Stationary phase occurs when a culture of bacteria exhausts
a required nutrient in its medium. The vast majority of cells in the population dies, but
the survivors adapt to this nutrient deprivation.
After screening more than 3500 isolates, we identified 123 clones that have a
lacZ reporter cassette inserted into a chromosomal gene that is induced by
stationary phase conditions. Inverse PCR with primers in the lacZ gene
followed by DNA sequencing were used to locate the open reading frames containing
the cassettes in these strains. Nearly three dozen strains that show very strong
induction in stationary phase have the cassette in genes with high homology to the
pilT and pilU genes of Pseudomonas. These genes, which have not
previously been described in Acinetobacter, encode NTPases that are involved
in twitching motility and in the uptake of DNA in natural transformation. The next most
frequent location of the cassette insertion is in a homolog of the tolB gene, a
periplasmic transport system component that is known to be upregulated by iron
restriction during stationary phase in Pseudomonas. A gene homologous to the
pyrophosphohydrolase mutT has also been identified in several strains.
MutT prevents oxidative damage to DNA by degrading 8-oxodGTP before it
becomes incorporated into DNA. We have begun knock-out studies to determine if
these induced genes are required for survival during starvation. Among other genes
that have also been identified are ones encoding arginyl- and glutamyl-tRNA
synthetases, carbamoylphosphate synthetase, shikimate kinase, aldehyde
dehydrogenase, EF-B, and possible serine/threonine kinases, multidrug resistance
proteins, and penicillin-binding protein.
The following photomicrographs of an Acinetobacter culture at various times
after inoculation of fresh medium illustrate the changes that occur as cells adapt to
starvation conditions.
 | Fig. 1: Live (stained green) and dead cells (stained red) in a culture six hours after inoculation of 0.5ml of a cell culture 24 hours old into 5ml of fresh medium. Notice that the growing cells are rod-shaped. |
 | Fig. 2: Live/dead stained culture that is 24 hours old. Notice that most of the cells in this dense culture are dead; only about 1% of the cells are alive. This is a stationary phase culture in which some required nutrient has been exhausted. |
 | Fig. 3: Live/dead stain of a culture that is 48 hours old. At this time only about 0.1% of the cells are alive. Notice also that the live cells are smaller than the dead cells. This is one of the many adaptations that the live cells are making in response to starvation. |
 | Fig. 4: Live/dead stained culture that is a week old. Notice that the number of cells does not appear to have decreased significantly but that the live cells are now very different in morphology (almost spherical in shape) compared to the dead cells that persist in the culture. |
An excellent introduction to Acinetobacter baylyi is available from the French laboratory that sequenced the entire genome of the organism. The URL for the site is http://www.genoscope.cns.fr/externe/English/Projets/Projet_DY/overview.html.
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