Chromophore: phytochromobilin P His ser B N 1 ys Leu Thioether Cis isomer GIn linkage Red light converts cis to trans Pro R R 15 se 1 Trans isomer His Leu GIn igure 9-5 FIGURE 17. 4 Structure of the Pr and pfr forms of the chro mophore (phytochromobilin) and the peptide region bound to the chromophore through a thioether linkage. The chro- mophore undergoes a cis-trans isomerization at carbon 15 in response to red and far-red light. (After Andel et al. 1997.)Figure 9-5