Writing in Proceedings of the National Academy of Sciences, Diana Bratu and colleagues describe a way to trace the movement of messenger RNA (mRNA) in living cells (doi:10.1073/pnas.2233244100).
Every moment inside cells, thousands of proteins and RNA molecules shuttle from one location to another. By fusing cellular proteins to other, fluorescent proteins, their movements can be followed visually. But it is not possible to tag RNA in the same way. Bratu et al. have devised a different sort of tracking system to monitor the transport of specific RNA messages. They created fluorescent 'molecular beacons' — short stretches of nucleic acids that seek out and bind to complementary mRNA sequences.
Attached to one end of the beacon is a fluorophore; a fluorescence quencher is fixed to the other end. The single-stranded beacon normally folds back on itself, forming a double-stranded hairpin structure in which the quencher and fluorophore are held in close proximity. But when the beacon binds to its complementary sequence in the RNA message, it unfolds — the fluorophore is separated from the quencher and the mRNA lights up.
To test their tracking system, the authors designed beacons for the oskar mRNA of fruitflies. The oskar message encodes the Oskar protein, which is involved in patterning the developing fruitfly egg. It is produced in 'nurse cells', which nurture the developing eggs. After entering and traversing the egg, oskar mRNA accumulates at the posterior end.
Bratu et al. injected oskar-specific molecular beacons into a living egg to see whether the pattern of fluorescence would unfold as expected. As the picture shows, it did — the authors detected the oskar-specific signal (green) in the neighbouring nurse cells and at the posterior end of the egg, whereas the signal from a control beacon was located throughout the egg (yellow).