Orientation of the chromophore transition moment in the 4‐leaved shape model for pea phytochrome molecule in the red‐light absorbing form and its rotation induced by the phototransformation to the far‐red‐light absorbing form

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Abstract
Orientation of the chromophore transition moment in pea phytochrome (subunit molecular mass, 114 kDa) in the red‐light‐absorbing form (Pr) and its rotation induced by the phototransformation to the far‐red‐light‐absorbing (Pfr) form was studied by measuring linear dichroism of oriented phytochrome prepared by a ‘gel squeezing method’. The phytochrome in Pr showed a negative linear dichroism. On the contrary, the Pfr form showed positive linear dichroism. Assuming that the molecular axis for the two‐fold rotational symmetry in the ‘4‐leaved shape model’ proposed recently for the dimeric molecular structure of the phytochrome [(1989) FEBS Lett. 247, 139–142], is oriented in parallel with the direction of the external constraint, the average azimuthal angle between the molecular axis and the transition moment of the chromophore was calculated as 59° or 121° for Pr and 52° or 128° for Pfr. These results explain well the polarotropic responses by phytochrome observed in plant organisms.

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