US scientists have set DNA detection in a spin by exploiting one of nature's molecular motors.

Wayne Frasch and co-workers from Arizona State University, in Tempe, have used enzyme F1-ATPase as the engine of a new DNA detection device. ATPases catalyse adenosine triphosphate (ATP) decomposition to produce energy. F1-ATPase can use this released energy to spin - it can act as a rotary motor.

Frasch's device works by coupling gold nanorods with F1-ATPases bound to a surface. Two short, labelled DNA strands complementary to a target DNA sequence are added to a DNA sample. If the target DNA is present, the strands bind to it side-by-side, forming a stiff DNA bridge with labels at each end (see figure). The labels used are molecules of biotin (shown in blue), a vitamin that binds strongly to the glycoprotein avidin (shown in green), which is found in egg white. When a solution of the DNA is dropped onto a surface coated with avidin-modified F1-ATPases, the DNA bridges bind by one end to avidin units using one biotin. An avidin-coated gold nanorod is then bound to the other end of each bridge. The F1-ATPase is made to spin by adding ATP and the gold nanorods also spin, being attached through the bridges. This can be detected simply using microscopy.

The system's detection limit is fewer than 600 DNA molecules in solutions of femtomolar concentrations. Conventional fluorescence-based DNA detection systems have detection limits of only about five picomolar; when fewer targets are present, either multiple fluorescent molecules must be used for each target or DNA amplification, typically using the polymerase chain reaction (PCR), is needed to generate a detectable fluorescent signal. The nanorod sensing method 'avoids the problems inherent to PCR and is much faster than current assays,' said Frasch.

Ulf Landegren, an expert on DNA detection from Uppsala University, Sweden, said, 'the critical question is how the device performs under field or regular lab conditions, where PCR and its related variants have dominated so far.' Frasch admits that he has reported the results from 'clean systems', but his team is now repeating experiments using real samples.

According to the US scientists, their system lends itself to an easy kit-based protocol. But the really exciting thing, said Frasch, is that it is the first practical nanodevice that employs a molecular motor that really works.

Freya Mearns