Within the past years RNA interference (RNAi) has become one of the most valuable tools for post-transcriptional gene silencing. Making RNAi temporally and/or spatially controllable would even enlarge its scope of application. Attaching a lightremovable protection group to siRNAs is a very promising approach to achieve this control over RNAi. It has been reported that modifying siRNA nucleobases surrounding the mRNA cleavage site between the 10th and 11th nucleotides successfully suppresses RNAi. We investigated the influence of photolabile protection groups at these and the adjacent nucleobases on siRNA activity and chose to incorporate caged deoxynucleotides instead of ribonucleotides. The siRNAs designed by these means were shown to be completely inactive. By irradiation with UV light (366 nm) they could be fully reactivated and showed the same activity as their unmodified siRNA counterparts.

A little light release: Temporarily blocked (“caged”) dG nucleosides that can be incorporated into oligodeoxynucleotides are presented (see figure). Their hydrogen‐bond‐recognition site is inaccessible in the inactive form; this prevents, for example, the formation of secondary structures. Light can trigger the release of the unmodified oligonucleotide and thus induce correct folding.

We have synthesized molecular beacons with caged nucleobases in the loop region. These constructs remain non-fluorescent in the presence of their target RNA but can be fully activated by light (366 or 405 nm) in vitro and in HEK293 cells. We suggest that this technology will be useful for single molecule RNA tracking in living cells.

The cover picture shows in the magnifying lens oligodeoxynucleotides that have been modified with a photolabile group, represented as a green star. This temporary modification blocks the interaction properties of the respective nucleobase. One of these "caged oligodeoxynucleotides" has just been hit with light, thus removing the modification. The unmodified oligodeoxynucleotide can now fold into the active conformation-in this case a G-quadruplex. The background suggests possible applications. Confocal microscopy can be used to apply light with high spatiotemporal control and allow the triggering of biologically active oligodeoxynucleotides; in the future maybe even in individual cells. For more details, see the communication by Mayer, Heckel, et al. on p. 1966 ff.