Can activate transcription (By similarity). Essential for light-regulated PHYA nuclear accumulation and subsequent PHYA phototropic signaling processes (PubMed:21969386, PubMed:22374392, PubMed:17566111). PHYA-specific signal transducer in response to continuous FR lights. Mediates the association of PHYA with HFR1 and LAF1 in the nucleus in response to FR conditions (PubMed:19482971, PubMed:16045472). Contributes to inhibition of hypocotyl elongation in continuous blue light (B) (PubMed:16045472). Homodimer and heterodimer with FHY1 (PubMed:16045472). Interacts with PHYA, especially upon far-red (FR) light illumination (PubMed:21969386, PubMed:21884939, PubMed:19482971, PubMed:19208901). Binds to LAF1 and HFR1 (PubMed:19482971). Shuttles from cytoplasm to nuclear bodies in FR but stay in the cytoplasm in other light conditions. Activated by FHY3/FAR1 under far-red light (FR); HY5 prevents this activation (PubMed:21097709, PubMed:18033885, PubMed:16045472). Down-regulated by FR, red (R) and blue (B) lights (PubMed:18033885, PubMed:16045472). Inactivated by rapid reversible PHYA-mediated phosphorylation. Partially blind to far-red (FR). Impaired inhibition of hypocotyl elongation and cotyledons expansion under continuous FR light conditions (PubMed:19482971, PubMed:16045472). In plants lacking FHY1 and FHL, altered phototropism (e.g. phototropic bending) associated with abnormal consitutive cytosolic localization of PHYA (PubMed:22374392, PubMed:17566111). In the double mutant fhl fhy1 several PHYA-dependent phototropic responses are altered (e.g. hypocotyl elongation and cotyledon opening under high-irradiance conditions and seed germination under very-low-fluence conditions), but not for some PHYA-dependent responses such as the abrogation of negative gravitropism in blue light and red-enhanced phototropism (PubMed:17566111). Hyposensitivity to blue light (B) (PubMed:16045472). Belongs to the FHY1 protein family.