Effects of Light，Oxygen，and Substrates on Steady-S...
The mRNA levels specific for ribulose-1，5-bisphosphate carboxylase，light-harvesting I polypeptides α andβ，and reaction center polypeptides L and M were assayed by use of a series of DNA probes specific for each cognate mRNA.Both the steady-state amounts and sizes of the specific mRNAs were measured as a function of the light intensity incident to the culture，the presence or absence of oxygen，and the type of substrate present in the growth medium.Northern hybridization revealed at least two and possibly three transcripts for ribulose-1，5-bisphosphate carboxylase.The cellular level of mRNA specific for ribulose-1，5-bisphosphate carboxylase increased in consort with enzyme activity as a function of both light intensity and reducing state of the substrate.Neither mRNA nor enzyme activity was detectable in aerobically grown cells.For the light-harvesting I and reaction center polypeptides there exist two transcripts，the larger of which appears to be a polycistronic mRNA possessing information for all four polypeptides and a smaller transcript specific for only the a andβ polypeptides of the light-harvesting I complex.The regulation of each of these mRNAs was affected by light and oxygen，but was not significantly affected by the oxidation-reduction state of the substrate.
The process of photosynthesis in photosynthetic organisms，whether plants，algae，or procaryotes，is strongly affected by environmental factors.Among these fators，light is certainly one of the most important.The regulation of photosynthetic genes by light has been well established in green plants，algae，blue-green bacteria，and photosynthetic bacteria［1，7，25］，but the molecular basis for this regulation is largely unknown.
此文发表于1985年“J.Bacteriol”162：925-932.
The purple nonsulfur photosynthetic bacterium Rhodopseudomonas sphaeroides provides an attractive system for studying the regulation of the activity of photosynthetic genes by light，O2，and other environmental parameters.Upon the removal of oxygen，the cell develops an extensive intracellular membrane that comprises the photosynthetic apparatus［5，6，11，12，16，17］.Further，the amount of intracellular membrane per cell and the composition of the intracellular membrane are functions of the light intensity［11，16，17］.Therefore，the synthesis and composition of the intracellular membrane are under at least two different levels of control，the p02 and the light intensity.(https://www.daowen.com)
In addition to the photosynthetic membrane components，ribulose-1，5-bisphosphate（RuBP）carboxylase（RuBPCase）in R.sphaeroides，a key enzyme of the Calvin cycle and a major soluble protein in the cytoplasm［21］，represents another model system for studying the regulation of photosynthetic gene expression.Two structurally and immunologically distinct forms of RuBPCase，forms Ⅰ and Ⅱ，exist in R.sphaeroides［27］.Form I is composed of eight large and eight small subunits and resembles the eucaryotic enzyme in its kinetic parameters［27］.Form Ⅱ，which lacks small subunits，is reported to be a hexamer of large subunits and resembles the procaryotic dimer enzyme as typified by Rhodospirillum rubrum［13，14，21］.The cellular levels of both forms of the enzyme are dependent on light，oxygen，and type of carbon corresponding author，substrate［29］.The light-and oxygen-dependent regulation of the synthesis of soluble and photosynthetic membrane proteins of R.sphaeroides is also clearly demonstrated by recent experiments involving shifts in these environmental parameters［5，7］.
With the advent of recombinant DNA techniques，DNA segments defining many of these important membrane and soluble proteins have become available.The authors of a recent paper have begun to examine specific mRNA levels in a related organism，Rhodopseudomonas capsulata［8］.The availability of specific DNA probes defining the genetic regions for the RuBPCase form Ⅱ［22］and for both the lightharvesting polypeptides（LH-1）（P.Kiley and S.Kaplan，unpublished data），and the reaction center polypeptides（RC-L and RC-M）［30，31］of R.sphaeroides has made it possible to qualitatively and quantitatively probe specific mRNAs by hybridization with these unique cloned fragments.The study presented here demonstrates the identification and regulation in the levels of mRNA coding for RuBPCase，LH-1，RC-L，and RC-M polypeptides in R.sphaeroides grown under defined steady-state conditions of light，oxygen，and carbon source.
