Lens-based optical microscopy failed to discern fluorescent features closer than 200 nm for decades, but the recent breaking of the diffraction resolution barrier by sequentially switching the fluorescence capability of adjacent features on and off is making nanoscale imaging routine. Reported fluorescence nanoscopy variants switch these features either with intense beams at defined positions or randomly, molecule by molecule. Here we demonstrate an optical nanoscopy that records raw data images from living cells and tissues with low levels of light. This advance has been facilitated by the generation of reversibly switchable enhanced green fluorescent protein (rsEGFP), a fluorescent protein that can be reversibly photoswitched more than a thousand times. Distributions of functional rsEGFP-fusion proteins in living bacteria and mammalian cells are imaged at <40-nanometre resolution. Dendritic spines in living brain slices are super-resolved with about a million times lower light intensities than before. The reversible switching also enables all-optical writing of features with subdiffraction size and spacings, which can be used for data storage.

VOLUME 29 NUMBER 10 OCTOBER 2011 nature biotechnology A r t i c l e sFluorescent proteins (FPs) 1 whose fluorescence can be reversibly or irreversibly switched by optical irradiation have opened new opportunities for the imaging of cells. They have facilitated in vivo protein-tracking schemes 2,3 , applications based on singlemolecule observations 4,5 and fluorescence microscopy with subdiffraction resolution [6][7][8][9][10] .Still, photoswitchable proteins have not displayed their full potential, because proteins that are just photoactivatable 11-13 can be switched only once, which implies that repeated measurements with the same molecule are impossible. On the other hand, photochromic or reversibly switchable fluorescent proteins (RSFPs) can be repeatedly photoswitched between a fluorescent and a nonfluorescent state by irradiation with light of two different wavelengths. However, in all previously characterized RSFPs, the wavelength used for generating the fluorescence emission is identical to one of the wavelengths used for switching the fluorescence on or off. The result is a complex interlocking of switching and fluorescence readout [14][15][16][17][18][19][20][21][22] , impeding or even precluding many applications, including fluorescence nanoscopy (super-resolution microscopy). Hence, the identification of an RSFP in which the generation of fluorescence is disentangled from switching has long been pursued. RESULTS Generation of the RSFP DreiklangNumerous GFP variants exhibit some degree of (generally undesirable) reversible photoswitching 4,23,24 . We found that the fluorescence of the yellow fluorescent protein Citrine 25,26 , a derivative of GFP, can be reversibly modulated to a small extent by alternate irradiation with light of 365 nm (on switching) and 405 nm (off switching), whereas fluorescence is excited at 515 nm. However, the achievable contrast was low, especially at pH values >6, rendering the reversible switching of Citrine unusable (Supplementary Fig. 1).To further develop this unusual switching behavior, we performed extensive random mutagenesis as well as directed PCR-mediated mutagenesis on a plasmid encoding Citrine. We transformed Escherichia coli with the plasmid, and screened with an automated home-built fluorescence microscope for bacterial colonies expressing fluorescent proteins whose fluorescence was excited with green light (515 nm) and which could be reversibly photoswitched from a fluorescent state to a long-lived nonfluorescent state by irradiation with near-UV (405 nm) light and back to a fluorescent state by UV (365 nm) light (Fig. 1a). In several consecutive screening rounds ~70,000 individual clones were analyzed. Finally, we identified a mutant differing from Citrine at four positions (Citrine-V61L, F64I, Y145H, N146D) ( Supplementary Fig. 2), which can be effectively switched and excited to fluoresce. We named this switchable fluorescent protein Dreiklang, the German word for a three-note chord in music.At thermal equilibrium, Dreiklang adopts the brightly fluorescent ...

The super-resolution microscopy called RESOLFT relying on fluorophore switching between longlived states, stands out by its coordinate-targeted sequential sample interrogation using low light levels. While RESOLFT has been shown to discern nanostructures in living cells, the reversibly photoswitchable green fluorescent protein (rsEGFP) employed in these experiments was switched rather slowly and recording lasted tens of minutes. We now report on the generation of rsEGFP2 providing faster switching and the use of this protein to demonstrate 25–250 times faster recordings.DOI: http://dx.doi.org/10.7554/eLife.00248.001

We show that nanoscopy based on the principle called RESOLFT (reversible saturable optical fluorescence transitions) or nonlinear structured illumination can be effectively parallelized using two incoherently superimposed orthogonal standing light waves. The intensity minima of the resulting pattern act as 'doughnuts', providing isotropic resolution in the focal plane and making pattern rotation redundant. We super-resolved living cells in 120 μm × 100 μm-sized fields of view in <1 s using 116,000 such doughnuts.

Smart fluorophores", such as reversibly switchable fluorescent proteins (RSFPs), are crucial for advanced fluorescence imaging. However, only a limited number of such labels is available and many display reduced biological performance compared to more classical variants.We present the development of robustly photoswitchable variants of EGFP, named rsGreens, that display up to 30-fold higher fluorescence in E. coli colonies grown at 37°C and more than 4-fold higher fluorescence when expressed in HEK293T cells compared to their ancestor protein rsEGFP. This enhancement is not due to an intrinsic increase in the fluorescence brightness of the probes, but rather due to enhanced expression levels that allow many more probe molecules to be functional at any given time. We developed rsGreens displaying a range of photoswitching kinetics and show how these can be used for multi-modal diffraction-unlimited fluorescence imaging such as pcSOFI and RESOLFT, achieving a spatial resolution of ~70 nm. By determining the first ever crystal structures of a negative reversibly switchable FP derived from Aequorea victoria in both the "on"-and "off"-conformation we were able to confirm the presence of a cis-trans isomerization and provide further insights into the mechanisms underlying the photochromism. Our work demonstrates that genetically encoded "smart fluorophores" can be readily optimized for biological performance, and provides a practical strategy for developing maturation-and stability-enhanced photochromic fluorescent proteins.KEYWORDS: fluorescent proteins, reversible photoswitching, super-resolution fluorescence microscopy, SOFI, RESOLFT, crystal structure determination, rsEGFP, superfolder Fluorescent proteins (FPs) enable the minimally-invasive labeling of intracellular structures in live systems. 1 The discovery and development of "smart photoactive FPs", 2,3 with features such as irreversible photoactivation and photoconversion, or reversible photoswitching, allowed the development of diffraction-unlimited imaging techniques such as (f)PALM 4,5 ((fluorescence) photoactivated localization microscopy), RESOLFT 6 (reversible saturable optical fluorescence transitions) and (pc)SOFI 7,8 ((photochromic) stochastic optical fluctuation imaging). These techniques strongly rely on the performance of the fluorophores and considerable efforts have therefore been dedicated to create optimized "smart labels". 9 This is exemplified by the continuous optimization and diversification of the EosFP family, 10-15 or the development of Dronpa 16 mutants with different or added photophysical properties. [17][18][19][20][21][22] Probes that combine multiple "smart" behaviors have also been engineered. [23][24][25] On the whole, however, the general acceptance of the FP-based "smart labels" has not quite risen up to the high expectations set by the many applications they enable. In some cases this is due to concerns surrounding the biological compatibility of the labels, meaning that the label may interfere with the functioning of the syst...