Posted: November 10, 2006

Virus detection with molecular machines

(Nanowerk News) Tiny machines that patrol the body for invaders are one of nanotechnology's favourite dreams. But a device made from a single molecule by a team of researchers in Israel sounds remarkably similar.
They have built a 'DNA machine' that detects a virus by reading its genome, and then produces an alarm signal, in the form of a visible glow.
But the machine is a far cry from a virus-zapping 'nanobot', not least because it needs a cocktail of other chemicals to function. The aim wasn't to make some kind of molecular robot, but to create a new way to spot traces of viruses in, say, a sample of blood.
Itamar Willner of the Hebrew University of Jerusalem and his co-workers say that their DNA device can provide a readout within an hour and a half, whereas existing methods for identifying viruses or bacteria from their DNA generally require many complicated chemical steps. They reported their results in Angewandte Chemie International Edition ("A Virus Spotlighted by an Autonomous DNA Machine ")
"This is a very sensitive method," says Chengde Mao, a specialist in DNA nanotechnology at Purdue University in West Lafayette, Indiana. "I think it is important work," he adds.
Willner and his colleagues exploited DNA's ability to act both as a genetic database and an enzyme-like catalyst that speeds up chemical reactions.
Their molecular machine is a single strand of DNA containing three segments. One (section A) senses viral DNA; another (section C) contains the instructions for making a catalytic DNA molecule, or DNAzyme. The third (section B) is where an enzyme attaches and cuts the DNAzyme free from the rest of the molecule.
The machine works like this. A separate DNA molecule called a hairpin, because its two ends stick together to form a loop, latches on to the pathogen's DNA at one end and to the machine's section A at the other. This triggers enzymes in the solution to build the DNAzyme encoded in section C, and then to snip it free.
A molecule known as hemin then activates the DNAzyme, which transforms another molecule called luminol, making it emit light.
It sounds complex, and it requires the presence of several enzymes, DNA building blocks, and other ingredients. But it all happens automatically. And once the viral DNA switches on the DNA machine, it makes and releases the light-inducing DNAzyme over and over, amplifying the signal.
The machine's inventors say that it can detect the DNA of a bacteria-attacking virus called M13 at a level equivalent to a bag of sugar dissolved in Lake Michigan. They report their results in Angewandte Chemie1.
Mao thinks that it will be some time before such machines can compete with existing technologies for amplifying DNA, which benefit from many years of refinement. "But in the long term, maybe," he says.
Source: (Philip Ball)