Partial Symmetrization of the Photosynthetic Reaction Center
Robles S.J., Breton J., Youvan D.C.
Graduate Program in Applied Biological Sciences, Massachusetts Institute of Technology,
Cambridge 02139.
The bacterial photosynthetic reaction center (RC) is a pigmented intrinsic membrane
protein that performs the primary charge separation event of photosynthesis, thereby
converting light to chemical energy. The RC pigments are bound primarily by two homologous
peptides, the L and M subunits, each containing five transmembrane helices. These alpha
helices and pigments are arranged in an approximate C2 symmetry and form two possible
electron transfer pathways. Only one of these pathways is actually used. In an attempt to
identify nonhomologous residues that are responsible for functional differences between
the two branches, homologous helical regions that interact extensively with the pigments
were genetically symmetrized (that is, exchanged). For example, replacement of the fourth
transmembrane helix (D helix) in the M subunit with the homologous helix from the L
subunit yields photosynthetically inactive RCs lacking a critical photoactive pigment.
Photosynthetic revertants have been isolated in which single amino acid substitutions
(intragenic suppressors) compensate for this partial symmetrization.