Phototropins are UV-A/blue light photoreceptors containing two flavin mononucleotide (FMN)–binding domains, light, oxygen and voltage (LOV)1 and LOV2, of which LOV2 is more sensitive toward light and more important for the physiological response compared with LOV1. Some physiological responses are plant phototropism, chloroplast migration and stomatal opening. Oat phototropin 1 together with light-dependent autophosphorylation shows a reduced electrophoretic mobility and reduced immunoreaction against a heterologous antiserum; both effects were suggested to be caused by phosphorylation at the same sites (M. Salomon, E. Knieb, T. von Zeppelin and W. Rüdiger  Biochemistry 42, 4217–4225). In this study, we show that both effects can be separated from each other: at low temperature, reduced immunoreaction preceded the mobility shift, and irradiation with UV-C light led to the mobility shift without the loss of immunoreactivity. We demonstrated that UV-C light at 280 nm, which does not match any absorption maximum of FMN, leads to autophosphorylation of phototropin. It is hypothesized that UV-C light causes differential activation of the LOV domains via energy transfer from aromatic amino acids.
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Vol. 81 • No. 1