Conclusion
In this chapter，we have attempted to provide an overview of the organization and expression of the genes for the photosynthetic apparatus in photosynthetic prokaryotes as revealed in combined studies utilizing genetics，biochemistry，and molecular biology.Over the past few years，several laboratories have made efforts toward isolating and sequencing the genes coding for the membrane components，LH and RC proteins，electron carriers，and cytoplasmic components.It can be predicted that in the future all of the genes involved in photosynthesis，including many genes for Bchl and Crt biosynthesis，will be isolated and sequenced.The function of the photosynthetic apparatus in higher organisms can be determined by comparing their DNA sequences to those of the photosynthetic prokaryotes，as was done forQB-binding protein and proteins involved in oxygen evolution.Another important direction in this field is to study the developmental and environmental regulation of photosynthetic genes in response to oxygen and light.It appears that transcriptional control plays a major role in most cases，although posttranscriptional and posttranslational regulation may also be important in some cases.Recently，such studies have been pursued extensively in R.capsulatus and R.sphaeroides.We have found that different photosynthetic genes can be regulated either negatively or positively by oxygen concentration and light intensity.However，the detailed events and the exact mechanism of oxygen and light regulation，and the interactions and communication among photosvnthetic genes are still obscure.We expect that in the near future more attention will be paid to characterizing the regulatory soquences and franscriptional machinery，to defining the moleadar mechanism of transcriptional and posttranscriptional regalation，and so investigating the interactions of regulatory proteins，DNA and mRNA such in formation will eventually facieitate the development of a biotechnology for increasing the efficiency of photosynthesis and food production.
Figure 6 Schematic diagrams illustrating transcriptional（TC），posttranscriptional，and posttranslational（TL）regulation of photosynthetic genes in R.capsulatus.In development of the photosynthetic apparatus photosynthetic genes are expressed coordinately and differentially in response to environmental signals such as oxygen and light intensity.Current information indicates three levels of regulation：transcriptional regulation of mRNA synthesis，posttranscriptional processing of mRNA，and posttranslational assembly and insertion of pigment-protein complexes.The photosynthetic apparatus in ICM is comprised of LH complexes（LH-Ⅰ，LH-Ⅱ），RC polypeptides（L，M，H），cyctochrome bc1complexes，and ATPase.The polypeptides LH-Ⅰ（β，α）；RC subunits L，M，and H，and LH-Ⅱ（β，α）are encoded by the puf，puh and puc operons，respectively.Bchl（B）binding to LH andRC is the pigment participating in light harvesting（in LH）and photochemical reaction（in RC）.A large number of enzymes involved in[1]Bchl biosynthesis are encoded by various bch genes.Pigments Crt（C）in LH and RC play a role both in light harvesting and in protection of the photosynthetic apparatus from photooxidation.The enzymes catalyzing Crt biosynthesis are encoded by various crt genes arranged as a cluster（see Figure 2）.rbcL is the gene coding for the large subunit of formⅡ RuBPCase，which is a cytoplasmic enzyme catalyzing carbon dioxide fixation in the Calvin cycle driven by reducing power NADPH and ATP derived from the photochemical reaction and coupled with ATPase in the ICM（see Figure 1）.Oxygen has a major effect on the transcription of photosynthetic genes.Under anaerobic conditions the photosynthetic genes are activated.The exact mechanism for this activation has not been determined.Some crt genes，such as crtA，are activated by oxygen（Zhu et al.1986a；Zhu and Hearst 1987）.Oxygen also affects the stabilty of mRNA，accelerating the degradation of some mRNA（i.e.，LH-Ⅱ）to produce nucleotides（NT）.It is not clear if oxygen acts on the translational process or assembly and stability of the proteins in the membrane.There is some evidence supporting the theory that oxygen can directly affect the activity of enzymes such as those catalyzing tetrapyrrole synthesis（not shown in this diagram）（Lascelles 1975）.The transcriptional，posttranscriptional，and posttranslational regregulatory sequences and transcriptional machinery，to defining the molecular mechanism of transcriptional and posttranscriptional regulation，and to investiguating the interactions of regulatory proteins，DNA，and mRNA.Such information will eventually facilitate the development of a biotechnology for increasing the efficiency of photosynthesis and food production.(https://www.daowen.com)
