Scientists in the US have developed a way to control DNA amplification by switching it on and off using ultraviolet (UV) light.

The polymerase chain reaction (PCR) is widely used to amplify small amounts of DNA by repeatedly copying the nucleic acid sequence using a polymerase enzyme. The enzyme builds the copies by adding nucleotides to short strands of DNA called primers, which must be added to the mixture to kick-start the PCR. 

To control the PCR, Alexander Deiters from North Carolina State University, Raleigh, and his colleagues incorporated bulky protecting groups called cages into the PCR primers, preventing the copying process. Deiters explained: 'The presence of the cages distributed evenly throughout the PCR primer is sufficient to prevent its annealing to the DNA template and its amplification.' Shining UV light on the caging groups removes them and enables PCR to start. 

The team was also able to switch PCR activity off with light by designing a self-complementary primer which folds to form a hairpin. 'By site-specifically installing caging groups it is possible to block hairpin formation,' said Deiters. The open-chain primer is free to bind to the DNA template, and PCR can occur. 'The PCR is stopped by removing the photoactive groups through irradiation,' Deiters explained.  

With the two caged primers possessing opposite effects, the researchers were able to use them simultaneously and stop the production of one PCR product, while triggering the amplification of another.

'We expect that the photoregulation of PCR will afford an additional level of control over this important technique,' said Deiters. 'PCR thermocyclers could be easily equipped with UV light sources enabling primer decaging at any time of the reaction, allowing for the design of PCR experiments which were previously not conceivable.'

Durjoy Majumder, an expert in gene expression from the Bengal Engineering & Science University, Shibpur, in Howrah, India, said that 'the technique has the possibility to assist crucial PCR-based biological investigation.' However, its application to genome analysis requires further experimentation, he added.

Michael Spencelayh