Bacteria see the light

Blue-light-sensing proteins unexpectedly help regulate virulence in at least one species of infectious bacteria, and the mechanism might prove common among other microbes, according to a study published this week in Science.

"There's a whole universe of bacterial photobiology to be uncovered," John Kennis at Vrije University in Amsterdam, who did not participate in this study, told The Scientist. Kennis cowrote a commentary published with the new findings.

About a decade ago, researchers found that certain plant photoreceptors contain protein modules called light, oxygen, or voltage (LOV) domains, sequences roughly 110 amino acids long that absorb blue light. Since then, these domains have been identified in nearly 100 bacterial species. But because many of them are not photosynthetic and had no known or expected response to light, the function of LOV domains was unknown.

Roberto Bogomolni at University of California, Santa Cruz, and his colleagues investigated LOV domains in Brucella abortus, a bacterium that causes abortions in livestock and fevers in humans. In this germ, the LOV domain is linked with a histidine kinase. Each histidine kinase phosphorylates a protein known as a response regulator. These interactions act as the primary system for signal transduction in bacteria.

The researchers grew wild-type Brucella and a mutant strain engineered with a disabled LOV histidine kinase in both lit and darkened lab conditions in the presence of cultured mouse macrophages. Wild-type Brucella grown in the light was roughly 10 times more virulent than wild-type Brucella grown in the dark and the strain without functioning LOV. In other words, depriving the bacterium of light led to similar results as disabling the enzyme, suggesting a link between LOV histidine kinase and the light-triggered rise in virulence.

The researchers also confirmed that LOV histidine kinases were sensitive to light in a plant microbe, a marine microbe and another Brucella pathogen, Brucella melitensis.

"Brucella has been known about for 120 years, and nobody in the world even suspected that it was sensitive to light," Bogomolni told The Scientist.

Further studies will have to determine factors such as the amount of light needed to activate the enzyme, the identity of its downstream partners, and how this mechanism protects the bacteria from macrophages. Bogomolni noted that a similar kind of light-sensitive protein module known as BLUF domains, present in many of the same organisms that contain LOV domains, may also play a role in such a signaling pathway.

The researchers suspect that after the bacterium gets ejected from its host - say, when a cow's infected fetus gets aborted and is lying in the field - sunlight helps increase Brucella's virulence using this LOV pathway. This could then make the germ more successful in infecting a new host.

The findings "open up a whole new suite of experiments people hadn't even considered before," Emmanuel Liscum at the University of Missouri-Columbia, who did not participate in this study, told The Scientist, noting that most researchers grow bacteria in dark incubators. "I suspect there'll be a huge explosion of data, since performing such experiments concerning dark and light are relatively simple to do."

LOV domains are also found in fungi and even in a few Archaea. The protein modules likely evolved in early bacteria and then were co-opted by plants, fungi and other bacteria, Liscum said.

Sean Crosson at the University of Chicago, also a coathor on the commentary accompanying the study, noted that LOV histidine kinase may be regulating virulence via numerous pathways. For instance, light-activated HOV histidine kinase could make Brucella more virulent by triggering formation of biofilm, or making the bacterium better at taking up a limiting nutrient, he told The Scientist.

While LOV histidine kinase helps Brucella regulate its virulence, "that might not be what it does in other bacteria," Crosson said. "There's a wide range of cellular functions these new photoreceptors could influence. This might be the tip of the iceberg."


Charles Q. Choi
mail@the-scientist.com

Links within this article:

T.E. Swartz et al. "Blue-Light-Activated Histidine Kinases: Two-Component Sensors in Bacteria," Science, August 24, 2007.
http://www.sciencemag.org

John Kennis
http://www.few.vu.nl/~john

J.T.M. Kennis and S. Crosson, "A Bacterial Pathogen Sees the Light," Science, August 24, 2007.
http://www.sciencemag.org

E. Huala et al., "Arabidopsis NPH1: a protein kinase with a putative redox-sensing domain," Science, December 19, 1997.
http://www.the-scientist.com/pubmed/9405347

G. Gadda, "Flavins: Photochemistry and Photobiology," E. Silva, A.M. Edwards, eds., Royal Society of Chemistry, Cambridge, July 18, 2006.
http://www.the-scientist.com/pubmed/17571886

Roberto Bogomolni
http://www.chemistry.ucsc.edu/faculty/bogomolni.html

J.P. Roberts, "Protein Phosphorylation," The Scientist, June 30, 2003.
http://www.the-scientist.com/article/display/13894

S. Pincock, "Use the force, bacteria," The Scientist, December 1, 2006.
http://www.the-scientist.com/article/display/36660/

Emmanuel Liscum
http://www.biosci.missouri.edu/liscum/LiscumLabPage.html

Sean Crosson
http://crescentus.bsd.uchicago.edu

N. Johnston, "Debaffling biofilms," The Scientist, August 2, 2004.
http://www.the-scientist.com/article/display/14868