Near-infrared optogenetic tools engineered from bacterial phytochromes

Near-infrared light is favorable for use in mammalian tissues due to its low scattering and low absorbance by hemoglobin, melanin and water. Therefore, optogenetic tools for light manipulation in mammals should have action spectra within the near-infrared tissue transparency window. Interestingly, natural bacterial phytochromes utilize the low molecular weight biliverdin, found in most mammalian tissues, as a photoreactive chromophore. Due to their near-infrared absorbance and availability of endogenous biliverdin chromophore, bacterial phytochromes are preferred templates for designing of optogenetic tools for noninvasive light-control of biochemistry and physiology of living animals. Moreover, bacterial phytochromes spectrally complement existing genetically encoded fluorescent probes and optogenetic constructs that operate in the visible spectral range. Several bacterial phytochromes were already developed into the near-infrared optogenetic tools. The design strategies, properties in vitro and in mammalian cells, and applications of these tools are discussed.