Four totally conserved glycines are involved in the packing of the two cytochrome b hemes, b L and b H, of the bc 1 complex. The conserved glycine 131 is involved in the packing of heme b L and is separated by only 3 Å from this heme in the bc 1 complex structure. The cytochrome b respiratory deficient mutant G131S is affected in the assembly of the bc 1 complex. An intragenic suppressor mutation was obtained at position 260, in the ef loop, where a glycine was replaced by an alanine. This respiratory competent revertant exhibited a low bc 1 complex activity and was affected in the electron transfer at the Q P site. The k min for the substrate DBH 2 was diminished by an order of magnitude and EPR spectra showed a partially empty Q P site. However, the binding of the Q P site inhibitors stigmatellin and myxothiazol remained unchanged in the suppressor strain. Optical spectroscopy revealed that heme b L is red shifted by 0.8 nm and that the E m of heme b L was slightly increased (+20 mV) in the revertant strain as compared to wild type strain values. Addition of a methyl group at position 260 is thus sufficient to allow the assembly of the bc 1 complex and the insertion of heme b L despite the presence of the serine at position 131. Surprisingly, reversion at position 260 was located 13 Å away from the original mutation and revealed a long distance interaction in the yeast bc 1 complex.