Allosteric communication between DNA-binding and light-responsive domains of diatom class I aureochromes.

The modular architecture of aureochrome blue light receptors, found in several algal groups including diatoms, is unique by having the LOV-type photoreceptor domain fused to the C-terminus of its putative effector, an N-terminal DNA-binding bZIP module. The structural and functional understanding of aureochromes' light-dependent signaling mechanism is limited, despite their promise as an optogenetic tool. We show that class I aureochromes 1a and 1c from the diatom Phaeodactylum tricornutum are regulated in a light-independent circadian rhythm.

Structure of a Native-like Aureochrome 1a LOV Domain Dimer from Phaeodactylum tricornutum.

Light-oxygen-voltage (LOV) domains absorb blue light for mediating various biological responses in all three domains of life. Aureochromes from stramenopile algae represent a subfamily of photoreceptors that differs by its inversed topology with a C-terminal LOV sensor and an N-terminal effector (basic region leucine zipper, bZIP) domain. We crystallized the LOV domain including its flanking helices, A'α and Jα, of aureochrome 1a from Phaeodactylum tricornutum in the dark state and solved the structure at 2.

Allosterically regulated unfolding of the A'α helix exposes the dimerization site of the blue-light-sensing aureochrome-LOV domain.

Aureochromes have been shown to act as blue-light-regulated transcription factors in algae in the absence of phototropins. Aureochromes comprise a light-, oxygen-, or voltage-sensitive (LOV) domain as a sensory module binding the flavin chromophore and a basic region leucine zipper (bZIP) domain as an effector. The domain arrangement in aureochromes with an N-terminal effector is inversed to other LOV proteins.

Proton transfer to flavin stabilizes the signaling state of the blue light receptor plant cryptochrome.

Plant cryptochromes regulate the circadian rhythm, flowering time, and photomorphogenesis in higher plants as responses to blue light. In the dark, these photoreceptors bind oxidized FAD in the photolyase homology region (PHR). Upon blue light absorption, FAD is converted to the neutral radical state, the likely signaling state, by electron transfer via a conserved tryptophan triad and proton transfer from a nearby aspartic acid.

Blue-light-induced unfolding of the Jα helix allows for the dimerization of aureochrome-LOV from the diatom Phaeodactylum tricornutum.

Aureochromes have recently been shown to act as blue-light-regulated transcription factors in the stramenopile alga Vaucheria frigida. They comprise a light-, oxygen-, or voltage-sensitive (LOV) domain as a sensory module with flavin mononucleotide (FMN) as a chromophore and a basic region leucine zipper (bZIP) domain as an effector. Aureochromes are the only members of a large LOV protein family, where the effector domain is located N-terminal to the sensor domain.

Photoreaction of plant and DASH cryptochromes probed by infrared spectroscopy: the neutral radical state of flavoproteins.

Flavoprotein radicals are important intermediates in many biochemical processes. In the blue light sensor plant cryptochrome, the radical state acts as a signaling state. An isolation and assignment of infrared bands of flavin radicals in the most relevant spectral region of carbonyl stretches is missing because of their overlap with absorption of water and the protein moiety.