A novel photosensor protein containing the LOV domain, aureochrome (AUREO),
was discovered in Vaucheria frigid. There are two types of aureochrome,
aureochrome-1 (AUREO1: 38.6 kDa) and aureochrome-2 (AUREO2: 39.2 kDa).
Both AUREO1 and AUREO2 are found in a stramenopile alga, and these proteins
have different functions. RNA interference experiments demonstrated that
AUREO1 controls branch development, whereas AUREO2 allows development of
a sex organ. This protein consists of a LOV domain at the C-terminal and
basic region/leucine zipper (bZIP) domains. Aligned sequences of the LOV
domain of AUREO are similar to LOV1 and LOV2 domains of phototropin, which
are well-studied proteins that behave as blue light receptors in plants,
but they lack a Glu-Lys salt bridge that is conserved in all other known
LOV domains. bZIP domains are known to form a-helical structures, and are
widespread among eukaryotes, where they serve as transcription regulators.
Hence, AUREO is a unique photosensor containing the bZIP domain. It has
been reported that the bZIP domain of AUREO may be S-type, because it possesses
a DNA binding site that recognizes the sequence TGACGT.
The photochemical reaction dynamics of the LOV domain of aureochrome-1 (AUREO1-LOV) and the LOV domain with the bZIP domain (AUREO1-ZL) were studied by employing the transient grating (TG) technique, using size exclusion chromatography to verify results. For both samples, adduct formation takes place with a time constant of 2.8 microsec. Although significant diffusion changes were observed both for AUREO1-LOV and AUREO1-ZL after adduct formation, the origins of these changes were significantly different. The TG signal of AUREO1-LOV was strongly concentration dependent. From analysis of the signal, it was concluded that AUREO1-LOV exists in equilibrium between the monomer and dimer, and dimerization of the monomer is the main reaction, i.e., irradiation with blue light enhances the strength of the interdomain interaction. On the other hand, the reaction of AUREO1-ZL is independent of concentration, suggesting an intraprotein conformational change occurs in the bZIP domain with a time constant of 160 ms. These results revealed the different reactions and roles of the two domains; the LOV domain acts as a photosensor, leading to a subsequent conformational change in the bZIP domain, which should change its ability to bind to DNA. A model is proposed that demonstrates how aureochrome uses blue light to control its affinity for DNA.