The xanthopsins: a new family of eubacterial blue-light photoreceptors.

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Abstract
Photoactive yellow protein (PYP) is a photoreceptor that has been isolated from three halophilic phototrophic purple bacteria. The PYP from Ectothiorhodospira halophila BN9626 is the only member for which the sequence has been reported at the DNA level. Here we describe the cloning and sequencing of the genes encoding the PYPs from E.halophila SL‐1 (type strain) and Rhodospirillum salexigens. The latter protein contains, like the E.halophila PYP, the chromophore trans p‐coumaric acid, as we show here with high performance capillary zone electrophoresis. Additionally, we present evidence for the presence of a gene encoding a PYP homolog in Rhodobacter sphaeroides, the first genetically well‐characterized bacterium in which this photoreceptor has been identified. An ORF downstream of the pyp gene from E.halophila encodes an enzyme, which is proposed to be involved in the biosynthesis of the chromophore of PYP. The pyp gene from E.halophila was used for heterologous overexpression in both Escherichia coli and R.sphaeroides, aimed at the development of a holoPYP overexpression system (an intact PYP, containing the p‐coumaric acid chromophore and displaying the 446 nm absorbance band). In both organisms the protein could be detected immunologically, but its yellow color was not observed. Molecular genetic construction of a histidine‐tagged version of PYP led to its 2500‐fold overproduction in E.coli and simplified purification of the heterologously produced apoprotein. HoloPYP could be reconstituted by the addition of p‐coumaric anhydride to the histidine‐tagged apoPYP (PYP lacking its chromophore). We propose to call the family of photoactive yellow proteins the xanthopsins, in analogy with the rhodopsins.