Phytochromes are photochromic biliproteins found in plants as well as in some cyanotrophic, photoautotrophic and heterotrophic bacteria. In many bacteria, their function is largely unknown. Here we describe the biochemical and spectroscopic characterization of recombinant bacterial phytochrome from the opportunistic pathogen Pseudomonas aeruginosa (PaBphP). The recombinant protein displays all the characteristic features of a bonafide phytochrome. In contrast with cyanobacteria and plants, the chromophore of this bacterial phytochrome is biliverdin IXα, which is produced by the heme oxygenase BphO in P. aeruginosa. This chromophore was shown to be covalently attached via its A‐ring endo‐vinyl group to a cysteine residue outside the defined bilin lyase domain of plant and cyanobacterial phytochromes. Site‐directed mutagenesis identified Cys12 and His247 as being important for chromophore binding and photoreversibility, respectively. PaBphP is synthesized in the dark in the red‐light‐absorbing Pr form and immediately converted into a far‐red‐light‐absorbing Pfr‐enriched form. It shows the characteristic red/far‐red‐light‐induced photoreversibility of phytochromes. A chromophore analog that lacks the C15/16 double bond was used to show that this photoreversibility is due to a 15Z/15E isomerization of the biliverdin chromophore. Autophosphorylation of PaBphP was demonstrated, confirming its role as a sensor kinase of a bacterial two‐component signaling system.

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Growth factors such as the neurotrophins promote neuronal survival and shape neuronal morphology. Neurotrophin receptors are located on the surface of axons and dendrites and must convey their signal retrogradely to the nucleus to influence transcription of target genes. The distance between the site of receptor activation and the nucleus is tremendous. How is the retrograde transmission of survival signals being achieved? Recent work showed that signaling endosomes containing neurotrophin receptors and associated downstream kinases undergo retrograde vesicular transport along microtubules, propelled by the molecular motor dynein. The next objective in the "neurotrophin receptor trafficking meets signal transduction field" will be to elucidate the traffic control mechanisms governing the directed movement of signaling endosomes. Much is already known on the trafficking of the receptor for epidermal growth factor, EGFR. We will summarize the known traffic control mechanisms for EGFR and hypothesize whether EGFR-relevant traffic control mechanisms might also be relevant for neurotrophin receptor traffic control. Moreover, we speculate about potential implications of neurotrophin receptor traffic jams for neurodegenerative diseases.

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