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Two enzymatic systems from the lactic acid bacterium Oenococcus oeni, isolated from wine, have been studied. The first one is the H+-ATPase for which the activity was characterized under various conditions of growth. The activity gradually increased by l.6 to 1.9-fold upon inoculation at pH 3.5. The H+-ATPase activity did not vary significantly in function of the growth rate or with and without malic acid. However, acidification of the medium in the absence of malic acid induced the activity by 1.5 to 2.2-fold depending on the initial pH. The partially cloned H+-ATPase genes shared high homologies with those from other bacterial F0F1-ATPases. A mRNA of about 7 kb was detected by Northern blot and its size suggests that the genetic organization of O. oeni atp operon is similar to most F0F 1-ATPases. Furthermore, the amount of atp mRNA was shown to increase in acidic conditions. O. oeni H +-ATPase activity was pH-inducible and regulation of the expression seems to occur at the level of mRNA synthesis. Thus, the results confirmed the proposed role of the H+-ATPase in acid tolerance in O. oeni. / The second system studied was a chromosome-encoded P-type ATPase (CopB) and its putative transcriptional regulator (CopR). The copB gene encodes a protein showing great similarities with other Cu2+-ATPases of the CPx-type family of heavy-metal ATPases like Enterococcus hirae copB. Another gene (copR) was found 250 bp upstream of copB and displays great similarities with proteins of the MecI/BlaI family of transcriptional regulators, including En. hirae CopY repressor. O. oeni was shown to be highly resistant to copper and growth occurred in up to 30 mM CuSO4. Northern blot analyses indicated that the amount of copB mRNA increased upon a 0.2 to 4.0 mM copper stress suggesting that expression of the enzyme might be regulated at the level of mRNA synthesis. Whether CopR is involved in this regulation remains to be determined, but the results suggest that copRB genes might be involved in copper resistance in O. oeni.