Fig. 4.

Proposed genetically-encoded near-infrared (NIR) probes based on bacterial phytochromes: (a) versatile two-domain short-NIR and three-domain long-NIR fluorescent proteins (FPs), photoactivatable (PA) and photoswitchable (PS) three-domain NIR fluorescent proteins, (b) two-domain biosensors for redox status and metal ions (Meⁿ⁺), split biosensors for protein interactions resulted from enzymatic modifications, such as phosphorylation (designated as P−), and insertion-based biosensors to detect analytes, and (c) optogenetic tools controlling enzymatic activities, open and closed states of ion channels, and gene expression via regulation of interaction between DNA repressor and gene promoter. The schematic illustration of the structural elements of BphPs corresponds to those shown in Figure 2a. Please see text for more details.