The GAL4(DBD)::PHYB hemidriver contains the DNA-binding domain from the Saccharomyces cerevisiae GAL4 gene ( SGDID:S000006169 ) fused via a flexible linker to an N-terminal fragment of the Arabidopsis thaliana PHYB gene (TAIR:26548); the PHYB fragment contains the PAS2-GAF-PHY motifs, but lacks its PAS and histidine kinase-related domains. GAL4(DBD)::PHYB forms one half of the 'PhotoGal4' split-GAL4 system: upon incorporation of its chromophore phycocyanobilin (PCB) and when stimulated by red light of approximately 630nm, the PHYB portion of the GAL4(DBD)::PHYB hemidriver undergoes a conformational change and is activated, allowing it to heterodimerize with a hemidriver containing a transcriptional activation domain fused to the phytochrome binding domain of the Arabidopsis thaliana PIF6 gene (TAIR:39734), such as VP16(AD)::PIF6 (heterodimerization occurs between the PHYB and PIF6 portions of the two hemidrivers). This reconstitutes a functional transcriptional activator that can drive transcription of a downstream gene of interest. The reconstituted transcriptional activator binds specifically to the UAS regulatory sequence due to the presence of the GAL4 DNA-binding domain in GAL4(DBD)::PHYB. Since transcription is only activated at the intersection of the expression patterns of the two hemidrivers, the PhotoGal4 system can be used to drive expression in a restricted pattern. In addition, the light-dependent activation of GAL4(DBD)::PHYB is reversible; upon illumination of far-red light (>730nm) the PHYB portion reverts to the inactive state and dissociates from the PIF6-containing hemidriver, providing fine spatiotemporal control. The PhotoGal4 system is designed for use in ex vivo culture of cells and tissues. The PHYB phycocyanobilin chromophore is not present in animal cells and must be supplied exogenously (FBrf0246250).