Formation origination of characteristic absorption peaks at 423 nm in Rhodobacter azotoformans

Abstract:

[Background] The characteristic peak at 420?425 nm was frequently observed in anoxygenic phototrophic bacteria and often assigned to carotenoid component(s). However, the absorption peak at 423 nm in Rhodobacter azotoformans R7 did not exhibit the characteristic absorption of carotenoid(s). [Objective] To elucidate the formation mechanism of the 423 nm absorption peak in strain R7. [Methods] Analyses by UV-VIS spectrophotometry, thin layer chromatography, high performance liquid chromatography, mass spectrometry, ultracentrifugation and ion exchange chromatography were conducted to investigate the formation origin of the 423 nm peak. [Results] A characteristic absorption maximum at approximately 423 nm was displayed in the in vivo absorption spectrum of strain R7 cultured in glutamate medium, while it was shifted to approximately 415 nm in the absorption spectrum of pigment extract. However, the cell growth and the amounts of bacteriochlorophyll (BChl) and carotenoid (Car) of strain R7 were significantly reduced by addition of glutamate, compared to those with the supplement of yeast. Pigment composition analysis showed that the 415 nm peak was only characterized by magnesium protoporphyrin IX monomethylester (MPE). MPE could locate on the intracytoplasmic membrane with an absorption peak at 423 nm. These results indicated that the 423 nm absorption peak was caused by MPE accumulation. The analyses of pigment protein complex (PPC) showed that three components of PPCs were detected in both yeast and glutamate cultures, however, only the absorption spectra of peripheral light harvesting complex 2 (LH2) and the reaction center (RC) of glutamate cultures gave the 423 nm characteristic peak. Collectively, strain R7 was capable of produce two different types of LH2; MPE locating on either LH2 or RC could form the characteristic peak of 423 nm. [Conclusion] The characteristic peak at 423 nm in strain R7 originated from MPE accumulation rather than carotenoid. The accumulated MPE inside cells could bind to LH2 and RC and locate inside the intracytoplasmic membrane. It is difficult to obtain a large amount of MPEs as it is a biosynthetic intermediate of BChl. Strain R7 was capable of accumulating MPE, therefore, the further study for biosynthesis regulation of MPE contributes to elucidate the novel mechanism of photooxidation and photoprotection in photosynthesis.