The lack of inhibition of nitrogenases of aerobic bacteria by the contact herbicide Dinoseb [proceedings].

gradient of acrylamide (10-16.5%, w/v). The modified polypeptides were detected by fluorography. The reaction-centre polypeptides have been identified on the gels of the whole membranes by comparison with purified reaction centres. Iodination of chromatophores from R. capsulata results in heavy labelling of the H-subunit (Fig. la). In contrast, the Hsubunit of R. sphaeroides is only very weakly labelled (Fig. 16). Previous studies with R. sphaeroides using antibodies to the reaction centres (Valkirs et al., 1976) and proteolysis (Hall et al., 1978) and with Rhodospirillum rubrum using radioiodination (Zurrer et al., 1977) and proteolysis (Oelze, 1978) showed that the H-subunit was exposed at the outside of chromatophores. The lack of labelling in the H-subunit with chromatophores of R. sphaeroides is not due to shielding by the ATPase (cf. Reed et al., 1975), since a similar labelling pattern is found with sonicated membranes (ATPase removed). This illustrates one of the problems with using a single localization technique, where the lack of labelling can be assigned to either an intramembrane location or an absence of available groups on the exposed parts of the polypeptide under study. In this case, since antibodies to the H-subunit of R. sphaeroides reaction centres react with chromatophores, we prefer the latter explanation. The M-subunits of the reaction centres of both R . capsulata and R . sphaeroides are labelled in chromatophores. Previous immunological studies with chromatophores from R. sphaeroides produced contradictory results. Steiner et al. (1 974) found no reaction of anti-(L + M subunits) with chromatophores, whereas Feher & Okamura (1976) reported strong cross-reactions with both sides of the membrane. The incorporation of radioactivity into spheroplasts was rather low (only 30% of the specific activity obtained with chromatophores) and so interpretation of the spheroplast results is not reliable. However, it seems likely that the slight labelling of the Hand M-subunits in the spheroplasts reflects the presence of a few broken vesicles. We conclude from the present study that the Hand M-subunits are exposed at the cytoplasmic side of the membrane. In the absence of positive data on the L-subunit, its location remains unassigned.