Nine large-scale symmetry reaction center mutants were constructed in Rhodobacter capsulatus by replacing segments of the M subunit gene with the homologous region of the L subunit gene. Between them, the mutations resulted in symmetrization of essentially the entire region from the carboxy terminal portion of the C helix through most of the E helix. The amino acids in this region define about 80% of the environment of the reaction center cofactors. These studies show that roughly 80% of the amino acids that come in close contact with the cofactors involved in initial electron transfer can be made symmetric in a piecewise manner without loss of the ability to grow photoheterotrophically. However, the amino acid regions near the quinones and iron atom are much more sensitive to symmetrization and most of the large-scale changes in this region resulted in the loss of photosynthetic viability, probably due to loss of stable reaction centers from the photosynthetic membrane. More detailed analysis of the isolated photosynthetic membranes from these mutants showed that in all cases but one, there was some amount of charge separation occurring in the mutant reaction centers. This bank of mutants serves as a useful starting point for more detailed studies of the differential molecular interactions which occur between the two reaction center subunits and their associated cofactors.