THE ESSENTIAL ARGININE RESIDUE AT POSITION-210 IN THE ALPHA-SUBUNIT OF THE ESCHERICHIA-COLI ATP SYNTHASE CAN BE TRANSFERRED TO POSITION-252WITH PARTIAL RETENTION OF ACTIVITY

The substitution of arginine at position 210 in the a subunit of Esche richia coli F0F1-ATPase by either lysine or alanine causes dominance i n complementation tests with a chromosomal c subunit mutation. Reversa l of dominance was achieved for the a R210K mutation but not for the a R210A mutation by the presence of an aspartic acid residue at positio n 50 or at position 252 in the a subunit. It was concluded that positi on 210 in putative helix 4 of a previously proposed model of the a sub unit is close to position 252 in putative helix 5 and to position 50 i n putative helix 1. The juxtaposition of residues 252 and 210 was also indicated by the observation that the double mutant a R210Q/Q252R was partially functional. A revertant of the partially functional double mutant, isolated on succinate medium, was found to contain a third mut ation resulting in Pro-204 in the a subunit being replaced by threonin e. That the revertant phenotype was due to the a P204T change was conf irmed by site-directed mutagenesis. ATP synthesis in the revertant str ain was at near normal levels as judged by growth yield experiments, b ut the revertant strain was unable to pump protons in response to ATP hydrolysis.