NONNATIVE INTRAGENIC REVERSIONS SELECTED FROM SACCHAROMYCES-CEREVISIAE CYTOCHROME B-DEFICIENT MUTANTS - STRUCTURAL AND FUNCTIONAL FEATURES OF THE CATALYTIC CENTER N-DOMAIN

A total of 110 revertants have been isolated from two well characteriz ed cytochrome b deficient (mit(-)) mutants. The mit(-) mutations are l ocated in an extramembranous loop linking the transmembrane alpha-heli ces IV and V of cytochrome b which has been postulated to be part of t he catalytic center Q(N) and therefore is assumed to be essential for the functioning of the bc(1) complex. The molecular bases of the rever sions were identified by sequencing the cytochrome b mRNAs. This allow ed us to identify seven new structures of cytochrome b which are more or less compatible with its catalytic activity. The secondary mutation s occurred either at the level of the original site mutation or at adj acent positions (region 204-208 of the polypeptide chain), or even at a distance of more than 150 amino acids (position 30) suggesting topol ogical interaction between these two areas. All the revertants recover ed 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 bc(1) complex functioning induced by the mutation S206L and its restoration by non native revers ions are tentatively explained.