Specific inhibition of antenna bacteriochlorophyll synthesis in Chlorobium vibrioforme by anesthetic gases

Abstract

The green sulfur bacterium Chlorobium vibrioforme contains two types of bacteriochlorophyll (Bchl). The minor pigment, Bchl a, is associated primarily with the cell membrane and its reaction centers; and the major light-harvesting antenna pigment, Bchl d, is found primarily in the chlorosomes, which are attached to the inner surface of the cell membrane. Anesthetic gases, such as N20, ethylene, and acetylene, were found to inhibit the synthesis of Bchl d, but not of Bchl a, thus allowing the cells to grow at high light intensities with a greatly diminished content of antenna pigment. Chlorosomes were absent or sparse in inhibited cells. Porphyrins accumulated in the inhibited cells. The major one was identified as the Bchl precursor magnesium-protoporphyrin IX monomethyl ester (Mg-PPME) by comparative absorption and fluorescence spectroscopy and thin-layer chromatography of the porphyrin and its derivatives with those of authentic protoporphyrin IX. Small amounts of Mg-PPME were present in control cells, but the addition of inhibitor caused a rapid increase in the Mg-PPME concentration, accompanying the inhibition of Bchl d synthesis. Cells grown in the presence of ethephon (as a source of ethylene) and allowed to stand in dim light for long periods accumulated large amounts of PPME and other porphyrins and excreted or released porphyrins, which accumulated as a brown precipitate in the culture. Inhibition of Bchl d synthesis was relieved upon removal of the inhibitor. These results suggest that the gases act at a step in pigment biosynthesis that affects the utilization of Mg-PPME for isocyclic ring formation. Synthesis of Bchl d and Bchl a may be differentially affected by the gases because of compartmentation of their biosynthetic apparatus or because competition for precursors favors Bchl a synthesis. An ethephon-resistant mutant strain was isolated by selection for growth in dim, long-wavelength light. The mutant cells were also resistant to acetylene, but not to N20. The ability to reversibly generate viable Chlorobium cells that lack antenna pigments may be useful in photosynthesis research. The ethephon-and acetylene-resistant strain may be useful in the study of the enzymes and genes that are involved in the biosynthetic step that the gases affect.The green sulfur bacterium Chlorobium vibrioforme contains very large amounts of bacteriochlorophyll (Bchl) provided they receive sufficient light. Cells from such cultures appeared to contain few or no chlorosomes (Ormerod and Nesbakken, Proc. IV Int. Symp. Photosynthetic Prokaryotes).These observations are significant in several respects. First, the experimental system should be useful for investigating chlorophyll synthesis; second, the inhibition may also be related to the mechanism of action of gaseous anesthetics in animals, which is poorly understood at present; third, since the inhibition appears to be quite specific, it provides a method of growing cultures of C. vibrioforme with drastically reduced amounts of antenna Bchl, thus permitting spectrosc...

Anaerobic protoporphyrin biosynthesis does not require incorporation of methyl groups from methionine

“…However, as for C. vibrioforme, 14 C was not present in the porphyrin nucleus of MP to the extent required to support the hypothesis that porphyrin-methyl groups are derived from methionine-methyl.…”

“…The identity of this peak was not investigated further. However, on the basis of its incorporation of non-base-hydrolyzable 14 C from [ 14 C]methyl-L-methionine and its fluorescence emission spectrum, it is reasonable to conclude that it consisted of, or was derived from, chlorophyllides obtained from base hydrolysis of chlorosome chlorophylls (bacteriochlorophylls c and d) which are the major pigments of C. vibrioforme. These pigments absorb light maximally at 650 to 660 nm and contain as many as five L-methionine-derived nonester methyl groups (19).…”

“…Pigment extraction. C. vibrioforme pigments were extracted as described by Ormerod et al (14). A 60-ml culture was harvested by centrifugation (8,000 ϫ g, 5 min), and the cells were resuspended in 6 ml of cold acetone and then resedimented.…”

Chlorobium tepidum Mutant Lacking Bacteriochlorophyll c Made by Inactivation of the bchK Gene, Encoding Bacteriochlorophyll c Synthase

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Effect of carotenoid deficiency on cells and chlorosomes of Chlorobium phaeobacteroides

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