Mutational replacements of conserved amino acid residues in the beta subunit resulted in defective assembly of H+-translocating ATPase (F0F1) in Escherichia coli.

Mutant genes for the beta subunit of H+-translocating ATPase (F0F1) were cloned from Escherichia coli strains isolated in this laboratory. Determination of their nucleotide sequence revealed four missense mutations (strain KF39, Glu-41----Lys; strain KF16 and KF42, Glu-185----Lys; strain KF48, Gly-223----Asp; KF26 and 4 other strains, Ser-292----Phe). Two nonsense mutants (strain KF40, Gln-361----end; strain KF20, Gln-397----end) were also identified. Glu-41, Glu-185, and Ser-292 are conserved in the amino acid sequences of the beta subunits so far studied, and Gly-223, Gln-361, and Gln-397 are conserved in beta subunits from bacteria and mitochondria, but not in those from chloroplasts. The amounts of F1 subunits in the membranes of these strains were studied by immunochemical assay and two-dimensional gel electrophoresis. In the mutants studied, the amounts of alpha and beta subunits in the membranes were 69-21 and 46-2%, respectively, of the amounts in wild-type membranes, the amount depending on the strain. No delta and epsilon subunits were detected in membranes of a missense mutant KF16, although reduced amounts of alpha and beta subunits could be detected, suggesting that the F1 portion may not be connected to F0 through the delta and epsilon subunits. The altered residues in missense mutants or missing domains in nonsense mutants may be important for the subunit-subunit interactions or assembly of the entire complex. Genetic experiments on introduction of suppressor tRNA into strains KF40 and KF20 suggested that F1 could be active even when residue 361 or 397 was replaced by a Ser, Leu, or Tyr residue.