Heme and Bacteriochlorophyll BCHL Synthesis in Rhodopseudomonas spheroides

Control of Heme and Bacteriochlorophyll (BCHL) Synthesis in Rhodopseudomonas spheroides 

This photosynthetic organism, when grown aerobically in the dark on a malate-glutamate growth medium, forms heme and has the metabolism of an aerobic bacterium. When grown in the light in the absence or at low concentrations of O2, the bacterium begins to make BCHL 

The ALA-synthetase from R, spheroides has been obtained in a partially purified form. The activity of this enzyme appears to be controlled by at least three mechanisms (Fig. 10). One is by feedback 

A second mechanism is by heme acting as repressor for the synthesis of ALA-synthetase (2). Data that suggested this mechanism were obtained from mutant 2-33 which formed heme in traces but possessed a level of ALA-synthetase 2.5 times that of the wild type that is, without heme repression more ALA-synthetase was made [Lascelles and Hatch, 145]. The following experiments suggested either repression or feedback inhibition by heme, (a) When a mutant that could not form heme was used, coproporphyrin accumulated that is, without heme there was no limitation of ALA production, (b) Decreasing the O2 of the medium resulted in an increase in ALA-synthetase or its activity one may interpret this result as a shunting of porphyrins into the Mg branch, thus decreasing the availability of porphyrins for heme 

When the cells are aerated, BCHL synthesis ceases within minutes. Because the activator lost activity almost completely by aeration in 10 minutes, the rapid cessation of BCHL synthesis may be due in part to aeration. As noted by Marriot et al., the significance of the presence of an activator in a photosynthetic form of this bacterium is puzzling, since it might have been expected that under photosynthetic conditions ALA-synthetase should have been fully active in order not to limit BCHL formation. 

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