11 April 2008

Bloodhound bacteria swim to the smell of pesticide

Scientists at the University of Atlanta, working under the leadership of Dr. Justin Gallivan, interfered with the navigation system of Escherichia coli, as a result of which the bacteria acquired the ability to move in the direction of higher concentrations of the widely used herbicide atrazine.

On the surface of E.coli there are receptor proteins that identify various chemical compounds, which ensures the ability of bacteria to move in the direction of increasing the gradient of a particular compound - to its source. Information about the recognition of the compound is transmitted along the bacterial cell to the flagellum, which begins to rotate and moves the cell in the desired direction.

This sequence of events can be changed by manipulating bacterial RNA. To do this, scientists used a strain of E.coli that does not have a gene that provides cell movement. A molecular switch (riboswitch) was embedded in the genome of such bacteria – an RNA fragment containing the missing gene and turned on only in the presence of atrazine. Its presence in the nutrient medium activated the riboswitch, and the modified bacteria began to move towards higher concentrations of the herbicide.

Some bacteria break down atrazine to intermediates, so scientists have created a riboswitch that reacts exclusively to unchanged atrazine molecules, and not to the products of its metabolism. Simultaneous embedding in E.coli of such a riboswitch and genes of bacteria feeding on atrazine will allow the creation of microorganisms that seek and break down the pesticide, thus cleaning the soil. The authors claim that using this approach, it is possible to create different variants of bacteria that feed on different compounds that are not included in the normal diet of E.coli.

However, there are some limitations. The surface receptors of bacteria recognize very small fluctuations in the concentrations of compounds in the environment, and to activate the riboswitch, the compound must penetrate into the bacterial cell, which reduces the sensitivity of bacteria to the concentration difference. In addition, the speed of movement triggered by the riboswitch in natural conditions can be significantly lower than in a gel-filled Petri dish.

The biggest advantage of using riboswitches is the speed of their development and modification of bacteria to "train" them to the right compound. Theoretically, it is possible to modify the genes of membrane receptors, giving them sensitivity to various compounds, but this approach takes a lot of time, depends on the unpredictability of folding protein molecules and in some cases leads to the appearance of receptors that are sensitive not to a particular substance, but to a whole spectrum of compounds.

At the same time, the number of possible riboswitch sequences is very limited, and if a riboswitch corresponding to a certain small molecule exists, it can be identified within the framework of a single experiment. The most effective riboswitches can be identified by selecting bacteria moving at maximum speed.

Ultimately, riboswitches can be used not only to stop and start the movement of bacteria. The authors hope to develop a method of using a bioswitch complex for sequential activation of several genes. This will allow E.coli to perform a series of sequential manipulations, for example, attach to small particles (or absorb them) in one place and release them in another.

Portal "Eternal youth" www.vechnayamolodost.ru based on the materials of Nature


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