Tuesday, April 08, 2008

PLoS Biology - Darwinian Evolution on a Chip

For evolution afficionados, there is a cool new paper in PLoS Biology on using a microfluidic chip to conduct in vitro evolution experiments.

The paper, by Brian Paegel and Gerry Joyce from the Scripps Research Institute
relies on computer control and microfluidic chip technology to automate the directed evolution of functional molecules, subject to precisely defined parameters. We used a population of billions of RNA enzymes with RNA-joining activity, which were challenged to react in the presence of progressively lower concentrations of substrate. The enzymes that did react were amplified to produce progeny, which were challenged similarly. Whenever the population size reached a predetermined threshold, chip-based operations were executed to isolate a fraction of the population and mix it with fresh reagents. These steps were repeated automatically for 500 iterations of 10-fold exponential growth followed by 10-fold dilution.
They have a nice figure summarizing the system which I show below --- and people should check out the article. Note - I can legally include this figure here because of the use of a broad Creative Commons License by PLoS Biology. Note - one of the stories I saw about this at Medgadget.Com also pointed out how they can use the article however they want because of where it was published. The power of true Open Access.

(A) The evolution chip is mounted on a temperature-controlled stage. Solutions containing polymerase enzymes (E) and mono- and oligonucleotide components (S) are delivered to the chip via capillary tubing and output to a pressure-controlled collection vial (O). A microscope objective is used to focus laser excitation (λex = 490 nm) on the microfluidic channel and to gather fluorescence (λem = 535 nm), which is detected with a confocal PMT. Valve actuation and fluid flow are controlled by six independent vacuum lines.

(B) The microfluidic device is shown with the active circuit filled with blue dye.

(C) The serial dilution circuit consists of a mixing loop with fluid flow channels (red), fluid access reservoirs (blue), and control valves (black). Fluid flow around the loop is controlled by three two-way valves (a, b, and c). Fluid access to the loop from the input reservoirs (RE and RS) and to the output reservoir (RO) is controlled by bus valves (in and out). The bus valves allow access when open, and prevent access while preserving fluidic continuity within the loop when closed.

(D) During operation of the circuit, the expanding RNA population is incubated while undergoing slow cyclic mixing until the fluorescence reaches a pre-determined threshold. Then an aliquot of the population is isolated between valves in and out as fresh solutions of E and Sin and out, and the aliquot is mixed with the fresh solutions by rapid serial actuation of valves a, b, and c. Open valves are indicated by filled circles; closed valves are indicated by a red X. are drawn into the loop and spent materials are delivered to the collection vial. Finally, the loop is sealed by closing valves

From: Darwinian Evolution on a Chip Paegel BM, Joyce GF PLoS Biology Vol. 6, No. 4, e85 doi:10.1371/journal.pbio.0060085

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