The Biological Role of the ζ Subunit as Unidirectional Inhibitor of the F 1 F O -ATPase of Paracoccus denitrificans

The biological roles of the three natural F F -ATPase inhibitors, ε, ζ, and IF , on cell physiology remain controversial. The ζ subunit is a useful model for deletion studies since it mimics mitochondrial IF , but in the F F -ATPase of Paracoccus denitrificans (PdF F ), it is a monogenic and supernumerary subunit. Here, we constructed a P. denitrificans 1222 derivative (PdΔζ) with a deleted ζ gene to determine its role in cell growth and bioenergetics. The results show that the lack of ζ in vivo strongly restricts respiratory P. denitrificans growth, and this is restored by complementation in trans with an exogenous ζ gene. Removal of ζ increased the coupled PdF F -ATPase activity without affecting the PdF F -ATP synthase turnover, and the latter was not affected at all by ζ reconstitution in vitro. Therefore, ζ works as a unidirectional pawl-ratchet inhibitor of the PdF F -ATPase nanomotor favoring the ATP synthase turnover to improve respiratory cell growth and bioenergetics.