Control of the formation of bacteriochlorophyll, and B875- and B850-bacteriochlorophyll complexes in Rhodopseudomonas sphaeroides mutant strain H5

Light and oxygen regulation of the synthesis of bacteriochlorophylls a and c in Chloroflexus aurantiacus

Control of the synthesis of bacteriochlorophylls (Bchls) a and c by light and oxygen was studied in Chloroflexus aurantiacus grown in batch or chemostat culture with serine as the growth-limiting substrate. For comparison, inhibition by gabaculine of the formation of selected tetrapyrroles was studied. The inhibitory effect of gabaculine decreased in the following order of tetrapyrrole formation: coproporphyrin greater than Bchl c greater than Bchl a. Not only did addition of 5-aminolevulinate (ALA) reverse the inhibition by gabaculine, it also caused an increase in Bchl c content when the cultures grew at high concentrations of ALA. Inhibition of Bchl a, Bchl c, and coproporphyrin formation by oxygen was similar to inhibition by gabaculine. Addition of ALA to aerated cultures led to significant accumulation of coproporphyrin. These results suggest that oxygen inhibits tetrapyrrole formation at a site before ALA formation. Control by light was studied with chemostat cultures transferred from 5 klx to 25 klx. This resulted in only a transient increase of the protein level of the culture, while specific contents of Bchls c and a and the ratio Bchl c/Bchl a decreased to lower steady states. However, the specific content of coproporphyrin increased. Addition of ALA to chemostat cultures adapted to 50 klx increased specific coproporphyrin and Bchl c contents by factors of about 20 and 4, respectively, while the specific Bchl a content was only slightly increased and protein levels were unaffected. Increasing the serine concentration caused an initial increase in the specific Bchl c content, which returned to the original value as soon as the protein content had attained its maximal level. These results suggest that light does not control ALA formation as strictly as oxygen and that competition of biomass formation and tetrapyrrole synthesis for common precursors may be influenced by light.

Inhibition of porphyrin biosynthesis by exogenous 5-aminolevulinic acid in an aerobic photosynthetic bacterium, Erythrobacter sp. OCh 114

Exogenously administered 5-aminolevulinic acid (ALA) inhibited the formation of bacteriochlorophyll a (Bchl a) in a dose-dependent manner in the aerobic photosynthetic bacterium, Erythrobacter sp. strain OCh 114, under dark growth conditions. The ALA concentration required for half-inhibition after 24-h growth was estimated to be about 3.0 mM. Porphyrin and Bchl precursors were not found in either the cells or the growth medium. The same inhibition was also observed with cytochrome c formation. When ALA was incubated with intact cells, a large amount of ALA was converted to an unknown metabolite. The pH optimum of the conversion was 7.8. The metabolite did not react with Ehrlich's reagent, but did so with ninhydrin, giving a yellow color. Based on analyses by several techniques including mass spectrometry, ir spectrometry, and paper electrophoresis, it was identified as 4-hydroxy-5-aminovaleric acid (HAVA). Authentic HAVA prepared from ALA was a competitive inhibitor of the enzyme, porphobilinogen synthase of Erythrobacter. The Ki value for authentic HAVA was calculated to be 2.4 mM from a Dixon plot and the HAVA concentration required for half-inhibition was 17 mM. It is concluded that in Erythrobacter cells, exogenous ALA is converted to the metabolite, HAVA, which is responsible for the inhibition of porphobilinogen synthase as well as that of Bchl a and cytochrome formation.

Regulation of tetrapyrrole synthesis by light in chemostat cultures of Rhodobacter sphaeroides

Control of bacteriochlorophyll (Bchl), magnesium protoporphyrin monomethyl ester (MgPME), cytochromes, and coproporphyrin by light was studied with chemostat cultures of Rhodobacter sphaeroides growing at a constant dilution rate. By increasing the growth-limiting light energy flux from 10 to 55 W/m2, specific Bchl contents decreased from 19.3 to 7.9 nmol/mg of protein. This was strictly proportional to a decrease in the ratio of B800-850 to B875 light-harvesting complexes. MgPME levels increased from 1.5 to 5.3 nmol/mg of protein, while cytochrome as well as coproporphyrin levels stayed constant at 0.46 and 1.95 nmol/mg of protein, respectively. Since in chemostat cultures steady-state levels of a product represent the rate of synthesis, these results infer only slight control of the rate-limiting step of total tetrapyrrol formation by light. In substrate-limited cultures MgPME was accumulated when growth and Bchl formation approached substrate saturation. This suggests that light controls a second step, i.e., MgPME conversion, whenever too much precursor is available, owing to the low sensitivity of the initial step of control. MgPME was preferentially localized in a subcellular fraction with high contents of B875 complexes. A second fraction exhibiting increased contents of B800-850 complexes lacked significant levels of MgPME. These results are discussed in terms of localization of Bchl synthesis in the membrane system of R. sphaeroides.

Molecular genetics of photosynthetic membrane biosynthesis in Rhodobacter sphaeroides

Images

Influence of pH, O2, and temperature on the absorption properties of the secondary light-harvesting antenna in members of the family Rhodospirillaceae

In some Rhodospirillaceae, the primary light-harvesting (LH I) antenna absorbs near-infrared light around 870 nm, whereas LH II (holochrome B800-860) has a major absorption band between 850 and 860 nm (B860) and a minor absorbancy around 800 nm (B800). Results show that, unlike LH I, holochrome B800-860 (LH II) exhibits unstable light absorption properties in whole cells. This was observed in Rhodopseudomonas capsulata grown anaerobically in light in weakly buffered carbohydrate medium; cultures lost both carotenoid-dependent brown-yellow pigmentation and LH II absorbancy. The whole cell spectrophotometric changes were attributed to mild acid conditions generated during sugar metabolism. LH II absorbancy was also destroyed in both R. capsulata and Rhodopseudomonas gelatinosa when cultures growing at neutral pH were acidified to a pH value around 5.0 with HCl. In contrast, during the same time period of exposure to pH 5.0, only a 50% decrease in Rhodopseudomonas sphaeroides LH II B800 absorbancy was measured. At neutral pH, LH II absorbancy in suspensions of nongrowing Rhodopseudomonas spp. was also sensitive to O2 exposure and to incubation at 30 to 40 degrees C. During treatment with O2, the rate of LH II B800 absorption decrease in R. gelatinosa and R. sphaeroides was 60 and 40% per h, respectively, compared with their absorbancy maximum around 860 nm. Both 860-nm absorbancy and the total bacteriochlorophyll content of the cells remained unchanged. On the other hand, no significant decrease in B800 if LH II in R. capsulata occurred during O2 exposure, but a 20% absorption decay rate per h of B800 was observed in cells incubated anaerobically at 40 degrees C. These B800 LH II spectral changes Rhodopseudomonas spp. were prevented by maintaining cells at neutral pH and at 10 degrees C. The near-infrared absorption spectrum of Rhodospirillum rubrum, which does not form LH II, was not significantly influenced by these different pH, aerobic, or temperature conditions.