A total of 110 revertants have been isolated from two well-characterized cytochrome b deficient (mit-) mutants. The mit- mutations are located in an extramembranous loop linking the transmembrane alpha-helices IV and V of cytochrome b which has been postulated to be part of the catalytic center QN and therefore is assumed to be essential for the functioning of the bc1 complex. The molecular bases of the reversions were identified by sequencing the cytochrome b mRNAs. This allowed us to identify seven new structures of cytochrome b which are more or less compatible with its catalytic activity. The secondary mutations occurred either at the level of the original site mutation or at adjacent positions (region 204-208 of the polypeptide chain), or even at a distance of more than 150 amino acids (position 30) suggesting topological interaction between these two areas. All the revertants recovered cytochrome contents and phosphorylation efficiencies similar to the wild-type ones, albeit differences appeared in their specific growth rates and NADH respirations. The failure in bc1 complex functioning induced by the mutation S206L and its restoration by non native reversions are tentatively explained.