Highly specific polyclonal and antibodies against either nitrate, nitrite or nitrous oxide reductases from a photosynthetic denitrifying bacterium Rhodobacter sphaeroides f. sp. denitrificans were used to show the presence of immunologically reactive proteins in strains that Pellerin and Gest had shown to grow in the dark with nitrate as a terminal acceptor [9]. Two strains of this bacterium, namely 81‐3 and 2.4.3 synthesized the three denitrifying enzymes and were capable of denitrification. Strains 81‐1 and 2.4.1 (neotype) both expressed nitrate reductase activities but nitrite reductase was not detected since these strains did not reduce nitrite. They also did not grow in the dark with nitrate as a terminal acceptor. Each of strains 81‐1, 81‐3, 2.4.1 and 2.4.3 contain four plasmids. R. sphaeroides f. sp. denitrificans, however, contains only one large 108 kb plasmid, which is distinctly different in size from those detected in the other strains. This indicates that the 108 kb plasmid is not necessarily specific for denitrification.

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The adaptation of cells of Rhodopseudomonas sphaeroides f. sp. denitriJicans for growth under denitrifying conditions in the light has been studied. The presence of nitrate in photosynthetically grown bacterial cultures resulted in a drastic reduction of carotenoid and bacteriochlorophyll contents as well as a loss of one of the polypeptides of the light harvesting complex, resulting in colour changes. Denitrifying cells had high activities of nitrate, nitrite and nitrous oxide reductases. The polypeptides corresponding to subunits of these enzymes were separated by PAGE. Synthesis of these enzymes was studied by pulse-chase labelling techniques. Nitrate and nitrite reductases are constitutive enzymes and it is likely that copies of mRNA for synthesis of these enzymes are 'long lived' in the cells.