46 THE BACTERIAL PHOTOCHEMICAL APPARATUS 



is accumulated by suspensions of Rps. spheroides incubated with 8- 

 azaguanine in the dark under low aeration (unpublished observations). 

 Griffiths (37) has recently isolated a series of mutants of i?/)S . 

 spheroides which do not form bacteriochlorophyll but which accumulate 

 a magnesium-free pigment with a spectrum similar though not identi- 

 cal with pheophorbide a. Its absorption maxima correspond with those 

 of Chlorohium 650 pheophytin. The slight differences in spectral 

 characteristics from pheophorbide a could denote differences in the 

 side chains of the dihydrotetrapyrrole nucleus though the data do not 

 show whether the compound is phytolated, 



Esterification withphytol. 



This may be the final stage in the synthesis of chlorophylls. In 

 higher plants short exposure to light results in formation of the un- 

 esterified chlorophyllide a, which is converted to the esterified 

 chlorophyll a by subsequent incubation in the dark (46). Accumulation of 

 the unphytolated-protochlorophyll pigment and of pheophorbide a by 

 mutants of Rps. spheroides suggests that esterification also occurs 

 at a late stage in bacteriochlorophyll synthesis. 



It seems likely that the esterification with phytol occurs within 

 the lipoprotein complex of the chromatophore because of the hydro- 

 phobic nature of the phytol residue. 



CONTROL OF TETRAPYRROLE SYNTHESIS 



It is obviously to the advantage of organisms which form chloro- 

 phylls to have mechanisms for regulating synthesis of the pigments to 

 fit environmental demands. Not only does the formation of chlorophylls 

 make a considerable drain on glycine and succinyl CoA but organisms 

 making these pigments must also elaborate a formidable array of 

 enzymes for the steps in the biosynthetic pathway. That control 

 mechanisms do exist is shown most clearly in the Athiorhodaceae, 

 which grow either aerobically in the dark or anaerobically in the light 

 and which form bacteriochlorophyll and carotenoids only under photo- 

 synthetic conditions. There is now evidence, though far from complete, 

 that synthesis of the pigments is subject to control by negative feed- 

 back mechanisms and by enzyme repression. 



Iron and hemin. 



The excretion of free porphyrins by photosynthetic bacteria under 

 conditions of iron deficiency shows a close resemblance to other bio- 

 synthetic systems where breakdown of a negative feedback control 

 occurs due to inability to form a metabolite which inhibits an early 

 enzymic reaction in the biosynthetic pathway (47). Thus, the accumu- 



