Transcription of Oxygen-Regulated Photosynthetic G...
Abstract
The regulation of the photosynthetic genes by DNA supercoiling in Rhodobacter capsulatus has been studied by using gyrase inhibitors in vivo and by measurement of mRNA levels of more than a dozen genes.The results demonstrate that the levels of mRNA for light-harvesting（Ⅰ，Ⅱ）and reaction center（L，M，H）proteins，bacteriochlorophyll biosynthetic enzymes，ribulose-bisphosphate carboxylase（EC 4.1.1.39），and the mRNAs from the open reading frames Q and R decreased immediately and dramatically upon addition of novobiocin and coumermycin.In contrast，the mRNAs for carotenoid biosynthetic enzymes，the cytochrome bc1complex，and constitutively expressed mRNA under aerobic conditions for light-harvesting I and for reaction center（L，M）proteins are less sensitive to the inhibitors.In accordance with these results，the biosynthesis of bacteriochlorophyll is markedly repressed by gyrase inhibitors novobiocin，coumermycin，nalidixic acid，and oxolinic acid either under anaerobic conditions or during a shift from aerobic to anaerobic conditions.The synthesis of light-harvesting（Ⅰ，Ⅱ）bacteriochlorophyll complexes is also inhibited by novobiocin and coumermycin.The kinetics of specific mRNA changes and the differential sensitivity of anaerobic and aerobic genes to the gyrase inhibitors strongly suggest that DNA supercoiling is involved in the differential expression of photosynthetic genes in response to the level of oxygen in R.capsulatus.
Yu Sheng Zhu，John E Hearst
Division of Chemical Biodynamics，Lawrence Berkeley Laboratory，and Department of Chemistry，University of California，Berkeley，CA 94720
Communicated by Martin Gellert，February 22，1988（received for review November 9，1987）(https://www.daowen.com)
此文发表于1988年“Proc.Natl.Acad.Sci.USA”85：4209-4213.
Rhodobacter capsulatus is a facultative purple nonsulfur Gram-negative bacterium capable of growing either aerobically in the dark（chemotrophic）or anaerobically in the light（phototrophic），primarily depending on the oxygen tension in the environment［1］.In the presence of oxygen，cells grow aerobically with an active respiratory electron transport chain as an energy source.Lowering the oxygen tension induces anaerobic growth with a photosynthetic electron transport chain producing energy.In the latter case the photosynthetic intracytoplasmic membrane composed of pigment-protein complexes including two light-harvesting（LH）antennae（Ⅰ，Ⅱ）and three reaction center（RC）polypeptides（L，M，H）develops.The pigments bacteriochlorophyll（BChl）and carotenoid（Crt）bound to the LH（Ⅰ，Ⅱ）polypeptides capture photons and funnel this energy into the RC.In the special membrane protein environment of the RC containing BChl，Crt，bacteriopheophytin，quinone，and iron，the charge separation occurs and the electron transport begins.The energetic photoelectron drives a cyclic series of redox complexes consisting of quinones and cytochromes（bc1，c2）in the membrane.The resultant proton gradient across the membrane drives ATP synthesis by way of an ATPase.The switch of the growth modes by oxygen is the result of activation and inactivation of a large number of genes responsible for different metabolic pathways［2］.
Earlier studies have indicated that the biosynthesis of Bchl in purple nonsulfur bacteria is inhibited by oxygen［3］.Several enzymes in the BChl biosynthetic pathway，such as δ aminolevulinate synthase，magnesium protoporphyrin methyltransferase［4］，and magnesium protoporphyrin chelatase［5］，appear to be repressed by oxygen.The transcription of a few genes in the Bchl biosynthetic pathway has been demonstrated to be regulated by oxygen using lac fusion［6］.The synthesis of pigment binding proteins involved in LH and the photochemical reaction in the photosynthetic membranes is also repressed by oxygen［7］.Recently，a number of studies［8-15］has shown that the levels of mRNA for LH（I，Ⅱ）and RC（L，M，H）complexes in R.capsulatus or Rhodobacter sphaeroides are reduced in response to oxygen.Although most photosynthetic genes are repressed by oxygen，the Crt genes，particularly the crtA gene，are activated by oxygen，perhaps for the protection of the cells from photooxidation［9］.It has been reported that an open reading frame（ORF）（designated as Q）upstream from the puf operon may function as a regulatory gene to control the puf operon and Bchl biosynthesis in response to oxygen［16］.Recently，the same authors have modified their hypothesis suggesting that the Q gene may code for a Bchl binding protein[3].We have found that two ORFs exist in the EcoQ fragment of R.capsulatus.The additional ORF［1473 base pairs（bp）］designated as R is located upstream from the Q gene（225 bp）with a space of 1 bp（M.Alberti and J.E.H.，unpublished data）.The R gene in R.sphaeroides seems to regulate the insertion of LH-Ⅰ in the membrane[4].Furthermore，we have found that the expression of both putative genes is also regulated by oxygen and light（see below）.This raises the further question：What is the fundamental mechanism for oxygen regulation？
Since most of the photosynthetic genes are coordinately expressed in response to the depletion of oxygen，there might be a common mechanism for their coordinate expression.Here we present in vivo evidence using the gyrase inhibitors novobiocin and coumermycin that the expression of the photosynthetic genes in R.capsulatus in response to oxygen is mediated by the supercoiling of DNA.
