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1 . Antibodies prepared against the Rhodospirillunt ruhrum F1-ATPase (RrFI) and its purified, native [hubunit, exhibited cross-reactivity with thc following soluble preparations of R. ruhrunt ATPase: RrFo . FI, RrFl and thc 8-subunit. Anti-RrF1, but not anti-l) antibodies, also formed precipitin lines with soluble @-less RrFl, indicating that antigenic determinants of both the 1-subunit and the other four RrFl-subunits are expressed in the whole RrFl molecule. Both antibodies agglutinated the R. rubvum chromatophores, suggesting that the [hubunit is located on the external part of RrF1.2. Both antibodies inhibited ATP synthesis and hydrolysis activities of R. rubrum chromatophores, as well a s all the soluble ATPase reactions. Similar concentrations of each antibody were required for 50 inhibition of all these reactions, but anti-RrFI was always somewhat more effective than anti-P. These data indicate that the []-subunit is involved in the catalytic site of the RrFl-enzyme.3. The antibodies prepared against R. rubrum F1-ATPase and its P-subunit could bind thc soluble chloroplast F1-ATPase (CF1) and inhibited ATP-linked reactions carried out by chloroplasts and by soluble CFI. In these reactions, unlike in the R. rubrun? ones, anti-@ was a more potent inhibitor than the anti-RrF1 antibody.The cross-reaction obtained between the antibodies raised against R. rubrurn F1 and its b-subunit and the chloroplast CF1 indicates the presence of similar antigenic determinants in the photosynthetic prokaryotic and eukaryotic F1-ATPases, which have been conserved during evolution.

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