Photosensors of “ Ca. S. mobilis”. (A) Domain structures of the four bacteriophytochromes encoded in the genome of “Ca. S. mobilis”. The experimentally verified red/far-red photocycle and BV chromophore of Cenrod_2641 are indicated by the darker colors; hypothetical photocycles are in faded colors. (B) Red/far-red photocycle of Cenrod_2641. The gene corresponding to Cenrod_2641 was co-expressed with heme oxygenase, which produces biliverdin (BV) from heme when expressed in E. coli. Purified Cenrod_2641 was characterized by absorbance spectroscopy before and after illumination with 700 ± 20 nm light. The (Before - After) difference spectrum is shown, with peak wavelengths indicated. (C) Dark reversion of the 750 nm photoproduct was characterized. Cenrod_2641 was held at photoequilibrium under 700 nm illumination. Illumination was then discontinued, and absorbance at 710 nm was monitored as a function of time. Data were biphasic, with the second phase only poorly resolved due to low signal-to-noise. We therefore fit the data to a single exponential with a linear second phase (equivalent to burst kinetics), deriving a rate constant of 0.2 s-1 for the fast initial phase. (D) Comparison of the dark-adapted absorption spectrum of Cenrod_2641 (red curve, arbitrary scale) to the absorption spectrum of intact consortia (black curve, arbitrary scale) and to the integrated number of accumulated consortia per 12 nm interval (bars, the scotophobotactic response of the consortia) (data from ). (E). Absorption spectrum of recombinant CpcA-PEB produced in Escherichia coli. BV, the precursor of phycoerythrobilin (PEB), was produced by Cenrod_2641 (heme oxygenase) and converted to PEB when cells were grown under oxic conditions. No red-colored/gold-fluorescent CpcA-PEB was produced under anoxic conditions, showing that the heme oxygenase encoded by Cenrod_2642 requires oxygen as a co-substrate for heme cleavage.