Complementation of the F o c Subunit of Escherichia coli with That of Streptococcus mutans and Properties of the Hybrid F o F 1 ATP Synthase

Abstract

ABSTRACT The c subunit of Streptococcus mutans ATP synthase (F o F 1 ) is functionally exchangeable with that of Escherichia coli , since E. coli with a hybrid F o F 1 is able to grow on minimum succinate medium through oxidative phosphorylation. E. coli F 1 bound to the hybrid F o with the S. mutans c subunit showed N,N′-dicyclohexylcarbodiimide-sensitive ATPase activity similar to that of E. coli F o F 1 . Thus, the S. mutans c subunit assembled into a functional F o together with the E. coli a and b subunits, forming a normal F 1 binding site. Although the H + pathway should be functional, as was suggested by the growth on minimum succinate medium, ATP-driven H + transport could not be detected with inverted membrane vesicles in vitro . This observation is partly explained by the presence of an acidic residue (Glu-20) in the first transmembrane helix of the S. mutans c subunit, since the site-directed mutant carrying Gln-20 partly recovered the ATP-driven H + transport. Since S. mutans is recognized to be a primary etiological agent of human dental caries and is one cause of bacterial endocarditis, our system that expresses hybrid F o with the S. mutans c subunit would be helpful to find antibiotics and chemicals specifically directed to S. mutans .