Organization and Expression of Genes for Photosynt...
Photosynthesis，the conversion by living organisms of light into chemical energy，is a fundamental biological process on earth.One of the major goals of plant biotechnology is to alter genetically the photosynthetic apparatus of crop plants to increase the efficiency of photosynthesis and ultimately to design an artificial photosynthetic system for harvesting solar energy.This strategy depends on a full understanding of the structure，organization，and function of the photosynthetic apparatus.Over the last few years a great deal of knowledge about the basic physics，chemistry，and molecular biology of photosynthesis has accumulated（Govindjee 1982；Sybesma 1984；Staehelm and Arntzen 1986；Steinback et al.1985）.However，the molecular events involved in biosynthesis，assembly，and regulation of the photosynthetic apparatus are not well understood.
Zhu Yu Sheng，Hearst John E.本文发表于1989年“Plant Biotechnology”（Kung，S.D.& .Arntzen C.J.eds），Chapter 11：257-287，Butterworth Publishers.
We are grateful to Dr.S.Kaplan for the use of his unpublished data.We thank D.Cook and G.Armstrong for their comments in preparation of this review.Part of this work was supported by the U.S.Department of Energy and by a grant from the National Institutes of Health.
Photosynthetic bacteria，genus Rhodobacter，including Rhodobacter capsulatus and Rhodobacter sphaeroides（formerly Rhodopseudomonas capsulata and R.sphaeroides，respectively）afford an unusual opportunity to study the molecular events of photosynthesis.These purple nonsulfur bacteria have a photosynthetic system that is less-complex than that of green plants and algae.Unlike higher photosynthetic organisms，they possess only one photosystem and lack the ability to oxidize water to oxygen.Their photochemical reaction center（RC）is similar to，but simpler than，that of photosystem Ⅱ，and can thereby provide fundamental information about the light reactions in more complex organisms（Kaplan and Arntzen 1982）.For example，a new model of photochemical function of photosystem Ⅱcomponents D1and D2 has resulted from a study of DNA sequence homology between genes coding for RC subunits L and M in R.capsulatus and genes coding for the 32-kd thylakoid membrane polypeptides in spinach and tobacco（Hearst and Sauer 1984a and 1984b；Hearst 1986）.Study of Rhodobacter has been greatly advanced by several well-developed genetic techniques，particularly for R.capsulatus，including the use of the gene transfer agent（Marrs 1974；Solioz et al.1975），marker rescue（Taylor et al.1983），transposon mutagenesis（Youvan et al.1982；Zsebo and Hearst 1984），and interposon mutagenesis（Scholnik and Haselkorn 1984）.Photosynthetic mutants are viable because of the remarkable metabolic versatility of purple nonsulfur bacteria.Rhodobacter can grow either chemoheterotrophically or photoheterotrophically，depending on oxygen tension and light intensity in the environment（Kaplan 1978；Drews 1978；Drews and Oelze 1981）.This dual lifestyle makes the organism an excellent laboratory tool with which to study the regulation of photosynthetic genes by light and oxygen as well as the formation and development of the photosynthetic membrane.Thus，in comparison with algae and higher plants，both the biophysical chemistry and the genetics of the photosynthetic apparatus are more advanced in the genus Rhodobacter.
The initial experiments that led to the discovery of the photosynthetic gene cluster of R.capsulatus were conducted by Marrs et al.They utilized the gene transfer agent，a particle resembling a generalized transducing phage，to map a gene cluster for bacteriochlorophyll（Bchl）and carotenoid（Crt）biosynthesis（Yen and Marrs 1976；Solioz et al.1975；Yen et al.1979）.Using the R factor mobilization technique，they isolated an R-prime plasmid，pRPS404，bearing 46 kb of chromosomal DNA that contained most of the photosynthetic genes of R.capsulatus（Marrs 1981）.This key step，achieved by genetic methods，is the basis for studying the molecular biology of photosynthesis in R.capsulatus.A subsequent breakthrough in the study of the photosynthetic genes of R.capsulatus was achieved by Hearst et al.when they reported the complete DNA sequence of the structural genes encoding the subunuits RC-L，RC-M，and RC-H（designated pufL，pufM，and puhA，respectively）as well as the light-harvesting（LH）I polypeptides LH-Ⅰβ and LH-Ⅰ α（pufB and pufA，respectively）（Youvan et al.1984a and 1984b）.Subsequently，the DNA sequence of two of the three LH-Ⅱ poly peptides，LH-Ⅱβ and LH-Ⅱ α（pucB and pucA，respectively），was determined by Youvan and Ismail（1985）.Additionally，17 genes for Bchl and Crt biosynthesis have been aligned with a detailed restriction map（Taylor et al.1983；Zsebo and Hearst 1984；Zsebo et al.1984）.Paralleling this achievement，the R.sphaeroides genes for RC-L，RC-M，RC-H（Williams et al.1983 and 1984；Donohue et al.1986a），LH-Ⅰ and LH-Ⅱ（Kiley et al.1987；Kiley and Kaplan 1987）have also been cloned and sequenced.Recently，the DNA sequences of the genes for cytochrome c2（cycA）（Daldal et al.1986；Donohue et al.1986b）and the iron sulfur ubiquinol be1，oxidoreductase complex（fbc）have been determined in both R.capsulatus and R.sphaeroides（Gabellini and Sebald 1986；Davidson et al.1987）.Even more recently the structural genes for the Form I ribulose bisphosphate carboxylase，phosphoribulose kinase，and cytochrome c'（cydA）have also been identified（Kaplan，personal communication）.Furthermore，the sequences of the genes for the B880 holochrome from Rhodospirillum rubrum（Bérard et al.1986）and the genes for H，L，and M subunits of the RC（Michel et al.1985 and 1986）from Rhodopseudomonas viridis have been determined.In summary，thus far the structural genes for LH complexes（except the γ subunit of LH-Ⅱ），RCs，and secondary electron carriers，as well as a few other genes involved in carbon dioxide fixation have been isolated and subjected to detailed structural analyses in both R.capsulatus and R.sphaeroides.(https://www.daowen.com)
At the protein level，Deisenhofer et al.（1985a）have recently presented a detailed crystal structure of the RC from R.viridis at 3 Å resolution.A three-dimensional structure of the R.sphaeroides RC（Allen and Feher 1984）has also been reported.This approach will lead to significant progress in our understanding of how protein structure facilitates primary photochemistry in the RC.
In addition to gene and protein structure，studies on transcriptional and posttranscriptional regulation of the photosynthetic genes，namely，the synthesis and processing of mRNA，are becoming an important branch in the study of the molecular biological properties of photosynthetic bacteria.Although early studies indicated the importance of transcriptional regulation（Biel and Marrs 1983 and 1985；Clark et al.1984；Klug et al.1984），the isolation and characterization by Northern blots and S-1 nuclease analysis of discrete mRNAs for LH-Ⅰ，RC-L，and RC-M were shown only by Belasco et al.（1985）for R.capsulatus，and by Zhu and Kaplan（1985）and Zhu et al.（1986b）for R.sphaeroides.Recently，we have described a more-detailed study of the regulation by oxygen and light of the genes coding for the structural proteins and pigment biosynthetic enzymes in R.capsulatus（Zhu and Hearst 1986；Zhu et al.1986a）.
In this chapter we have reviewed the recent progress in research aimed at the organization and regulation of photosynthetic genes primarily in R.capsulatus and R.sphaeroides.For other reviews regarding the biosynthcsis and assembly of the photosynthetic apparatus in purple photosynthetic bacteria，see the following articles：Drew 1978 and 1985；Kaplan 1978；Drews and Oelze 1981；Kaplan and Arntzen 1982；Youvan and Marrs 1984 and 1985；Drews et al.1986.
