Properties of the Mg Branch in Rhodopseudomonas spheroides

Methionine deficiency leads to coproporphyrin accumulation. Lascelles and Hatch [145] suggested that heme formation may be inhibited under these conditions, perhaps at the iron insertion step because methionine is required for the synthesis of phosphatidyl choline and the latter appears to be needed for ferrochelatase activity. Tait [147a] reported that under anaerobic conditions the conversion of coproporphyrinogen to protoporphyrinogen required methionine, ATP, and ferrous ions. 

The data available suggest the following hypotheses on the properties of the Mg branch. 

O2 blocks the transcription or translation of a block of genes coding for precursors of Mg porphyrins and photosynthetic apparatus. (This O2 action may be in addition to and independent of the action of O2 on the inhibition of the activator of ALA-synthetase.) The Mg branch must be independently controlled, since BCHL synthesis can be terminated without blocking heme synthesis. 

At least one of the enzymes of the Mg branch, possibly Mg chelatase, appears to have a short half-life, for when puromycin was given [145], heme continued to be formed for a while but BCHL synthesis ceased in a short time. 

At least the first enzyme of the Mg branch is present at limiting activities. Under photosynthetic conditions, with glycine and succinate as the organic components of the medium, the ratio of heme to BCHL was 1 40. When ALA was fed, the BCHL did not increase but the heme increased tenfold that is, the ratio of heme to BCHL became 10 40 [145]. This experiment suggested that the rate of BCHL synthesis is normally at a maximum in the photosynthetically growing cells and is not increased by ALA because Mg chelatase may be rate-limiting. 

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