Summary: We have compared the effect of diphenylamine (DPA) on the pigment composition of f. sp. grown in photoheterotrophic culture with the previously reported effect of nitrate. It was demonstrated that the effect of nitrate is due to the nitrite which is produced during denitrification. Both nitrite and DPA caused a decrease in the synthesis of spheroidene, and the accumulation of more-reduced precursors not normally seen. Nitrate (effectively nitrite) caused a decrease in the amount of bacteriochlorophyll, and the reaction centre from denitrifying cells did not contain the 28 kDal polypeptide (RC-H) subunit. These effects did not occur over a range of DPA additions (4 to 8 μ;g ml) to cells growing in the absence of nitrate. Denitrifying cells also had 40-50% lower activity of δ-aminolaevulinic acid synthase than those grown with or without DPA. Both nitrite and DPA treatments resulted in the loss of the B870 light-harvesting complex because of a failure to synthesize its 12 kDal polypeptide subunit.


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