The GAL4(DBD)::PHYB(Y276H) 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 and in addition contains a Y276H mutation. Upon incorporation of its chromophore phycocyanobilin (PCB), the PHYB portion of the GAL4(DBD)::PHYB(Y276H) hemidriver is able 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 and the presence of the Y276H mutation in the PHYB sequence results in the heterodimerization being independent of light stimulation. The reconstituted transcriptional activator can drive transcription of a downstream gene of interest and since transcription is only activated at the intersection of the expression patterns of the two hemidrivers, this can be used to drive expression in a restricted pattern. 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(Y276H). The PHYB phycocyanobilin chromophore is not present in animal cells and must be supplied exogenously (FBrf0246250).