Cell suspensions of mutant strains of Rhodopseudomonas spheroides, which cannot form bacteriochlorophyll, have been examined for their ability to form other tetrapyrroles under conditions of low aeration. With the exception of strain L-57, the mutants could form carotenoids. 2. All strains, like the parent organism, formed iron protoporphyrin when incubated with delta-aminolaevulate, showing that the iron branch of the biosynthetic pathway operated. 3. Magnesium protoporphyrin or its monomethyl ester was also formed from delta-aminolaevulate by all strains with the exception of L-57. 4. Coproporphyrin and coproporphyrinogen were accumulated by the parent and by strains 2/73 and 2/21 when incubated with glycine and succinate in the presence of ethionine. Strain 2/33, which required methionine for growth, accumulated these compounds in the presence and absence of methionine. 5. Strain L-57 did not accumulate porphyrins from glycine and succinate under any conditions. However, the delta-aminolaevulate synthase of this mutant showed the same rise in activity in response to reduced aeration as did that of the parent organism. 6. Ethionine inhibited production of protoporphyrin and its derivatives from delta-aminolaevulate by the parent strain. 7. The accumulation of coproporphyrin(ogen) under conditions of methionine deficiency may reflect the presence of enzymes of the magnesium branch of the biosynthetic pathway. Strain L-57 may lack a genetic element which determines the development of the entire photosynthetic apparatus. Since this strain did not accumulate coproporphyrin(ogen), the possibility of a specific delta-aminolaevulate synthase, directed towards bacteriochlorophyll synthesis, should be considered.
