Oxygen-Regulated mRNAs for Light-Harvesting and Re...
The stability and regulation by oxygen of mRNAs for the photosynthetic apparatus in Rhodobacter capsulatus have been studied by using proflavin to inhibit transcription and by shifting cells from anaerobic to aerobic conditions.The results from the inhibition experiments show that the mRNA for the light-harvesting LH-Ⅱ polypeptides（β，α）is more stable than that for the light-harvesting LH-Ⅰ polypeptides（β，α）during anaerobic growth，whereas the mRNAs for the reaction center polypeptides L（RC-L），M（RC-M），and H（RC-H）are less stable than both the LH-Ⅰ and LH-Ⅱ mRNAs.When photosynthetic cells are shifted from anaerobic to aerobic conditions，an immediate decrease in the levels of mRNA for the LH-Ⅰ，LH-Ⅱ，RC-L，RC-M，and RC-H proteins was observed.The level of mRNA for the LH-Ⅱ proteins，however，is more sensitive to oxygen and is reduced faster than the level of mRNA for the LH-Ⅰ proteins.These results suggest that oxygen represses the expression of genes coding for the light-harvesting antenna and reaction center complexes and may selectively accelerate the degradation of mRNA for the LH-Ⅱ proteins.The mRNAs for several enzymes in the bacteriochlorophyll biosynthetic pathway are regulated by oxygen in a similar manner.The mRNAs for carotenoid biosynthetic enzymes，however，are regulated by oxygen in a different way.We have found that the amounts of mRNAs for carotenoid biosynthetic enzymes，relative to the amounts of mRNAs for LH and RC，increased during the shift from anaerobic to aerobic conditions.We have particularly shown that although the expression of most photosynthetic genes in R.capsulatus is repressed by oxygen，the crtA gene，located in the BamHI H fragment of the R'plasmid pRPS404 and responsible for the oxidation of spheroidene to spheroidenone，responds to oxygen in an opposite fashion.This enzymatic oxidation may protect the photosynthetic apparatus from photooxidative damage.
Division of Chemical Biodynamics，Lawrence Berkeley Laboratory，and Department of Chemistry，University of California，Berkeley，California 94720
Received 23 June 1986/Accepted 9 September 1986
Corresponding author.(https://www.daowen.com)
此文发表于1986年“J.Bacteriol.”168：1180-1188.
Rhodobacter capsulatus［19］，which normally inhabits muddy lake bottoms and sewage lagoons，is a gram-negative，facultative，photosynthetic bacterium.It can generate metabolic energy by either aerobic respiration or anaerobic photosynthesis depending on the oxygen concentration and light intensity in its environment.The photosynthetic apparatus is composed of pigment-protein complexes for the conversion of light to chemical energy，including a shortwavelength light-harvesting antenna，LH-Ⅱ（also denoted B800-850 by its near-infrared absorption maxima），a long wavelength light-harvesting antenna，LH-Ⅰ（also denoted B870），and a reaction center complex consisting of three polypeptides（RC-L，RCM，and RC-H）.Each of these complexes binds bacteriochlorophyll and carotenoid pigments.The photosynthetic apparatus contains additional electron transport proteins to complete the cyclic pathway of electron transport.A decrease in oxygen concentration induces formation of the photosynthetic apparatus，which is localized within invaginations of the cellular membrane known as the intracytoplasmic membrane［16，20，26］.High levels of oxygen inhibit the formation of the photosynthetic apparatus，and under these conditions the cells use oxygen as a terminal electron acceptor for energy generation through aerobic respiration［16，17］.Oxygen concentration thus is one of the primary environmental factors governing the switch between alternative growth modes in R.capsulatus.However，the mechanism whereby oxygen controls photosynthetic gene expression is unknown.
The effects of oxygen on various photosynthetic functions have been well documented.Early studies by Cohen-Bazire et al.［12］demonstrated that oxygen represses bacteriochlorophyll biosynthesis in purple nonsulfur photosyn thetic bacteria.The control of bacteriochlorophyll formation by both oxygen and light has also been described［1，2，27］.The transcription of several genes for bacteriochlorophyll biosynthesis in R.capsulatus has been shown to be regulated by oxygen［4］.In addition，it has been stated that the carotenoid biosynthesis is not directly regulated by oxygen［5］.Recently，we［39-41］and others［3，8，21］have shown that the levels of mRNA for LH-Ⅰ，LH-Ⅱ，and RC complexes in Rhodobacter sphaeroides or R.capsulatus are regulated by oxygen.Furthermore，we have found that the levels of mRNAs for several genes coding for bacteriochlorophyll and carotenoid biosynthesis are also regulated by oxygen［39］.In this paper，we have conducted a shift experiment from anaerobic photosynthetic conditions to aerobic conditions to analyze the effects of oxygen on the stabilities of transcripts for LH and RC proteins and transcripts for bacteriochlorophyll and carotenoid biosynthetic enzymes.The results presented in this communication demonstrate that oxygen immediately represses the expression of the genes coding for LH-Ⅱ，LH-Ⅰ，RC-L，RC-M，and RC-H proteins and bacteriochlorophyll biosynthetic enyzmes.Oxygen also significantly stimulated the degradation of the mRNA for the LH-Ⅱ proteins.Furthermore，we have found that the crtA gene responsible for the oxidation of spheroidene to spheroidenone in R.capsulatus is oxygen activated.
