Regulation of Expression of Genes for Light-Harves...
Abstract
RNA levels were measured by blot hybridization to study the coordinate and differential expression of Rhodobacter capsulatus genes for light-harvesting I antenna proteins LH-I and LH-Ⅱ；reaction center（RC）polypeptides L，M，and H；and bacteriochlorophyll and carotenoid biosynthesis in response to light and O2.The genes for LH-Ⅱ α andβ subunits only have one transcript，0.5 kilobase（kb）long，whereas the genes for LH-Ⅰ have two transcripts（0.5 and 2.6 kb）.The small transcript（0.5 kb）is the mRNA only for LH-Ⅰβ and α polypeptides，whereas the large transcript（2.6 kb）codes for RC-L，RC-M，and theβ and α polypeptides of LH-Ⅰ，as well as the product of an unknown open reading frame designated ORF C2397.These five genes thus comprise a single operon（designated the puf operon）.The mRNA specifying the LH-Ⅱ polypeptides is more abundant，more sensitive to changes in O2 concentration，and shows a variation over a wider range than that of the mRNA for LH-Ⅰ，indicating that the genes for LH-Ⅱ and LH-Ⅰ/RC are regulated independently.The gene for RC-H（puhA）has at least two transcripts（1.2 and 1.4 kb）that initiate within ORF F1696 and respond differentially to light intensity.The expression of the genes coding for RC-L，RC-M，and RC-H is coordinately regulated by light intensity and O2 concentration.An increase in light intensity causes a decrease in the expression of the genes for LH-Ⅰ，LH-Ⅱ，and RC proteins.The genes coding for the enzymes in the bacteriochlorophyll biosynthetic pathways are regulated by light intensity and O2 in a manner similar to that of the genes for LH and RC proteins.The crt genes coding for the enzymes in carotenoid biosynthetic pathways，however，are regulated in an opposite fashion：high light intensity results in increased expression of crt genes.These results are interpreted based on the protective function of carotenoids under high light intensity in the presence of O2.
Yu Sheng Zhu and John E.Hearst.Division of Chemical Biodynamics，Lawrence Berkeley Laboratory，and Department of Chemistry，University of California，Berkeley，CA 94720 Communicated by Melvin Calvin，June 20，1986
此文发表于1986年“Proc.Natl.Acad.Sci.USA.”83：7613-7617.
Abbreviations：LH-Ⅰ and LH-Ⅱ，light-harvesting proteins Ⅰ and Ⅱ；RC，reaction center；Bchl，bacteriochlorophyll；Crt，carotenoid（s）；ORF，open reading frame；kb，kilobase（s）.
关键词：photosynthetic bacteria/Rhodopseudomonas capsulata/puf operon/polycistronic mRNA/photooxidative damage(https://www.daowen.com)
The purple nonsulfur photosynthetic bacterium Rhodobacter capsulatus［formerly Rhodopseudomonas capsulata［1］］provides an attractive model for studying the regulation of photosynthetic genes by light，O2，and other environmental factors.When the concentration of O2 is lowered，the cell develops an extensive intracytoplasmic membrane and induces the biosynthesis of light-harvesting LH-Ⅰ（B870）and LH-Ⅱ（B800-B850）and reaction center（RC）proteins，as well as bacteriochlorophyll（Bchl）and carotenoids（Crt）［2-4］.Once assembled，the photochemically active complexes harvest and convert light energy into chemical energy［5，6］.The size and structure of the photosynthetic apparatus are also influenced by light intensity［2-4，6］.
The genes for theβ（pu f B）and a（pu f A）subunits of the LH-Ⅰ complex，as well as the L（pu f L）and M（pu f M）subunits of the RC complex，are located in the Bam HI-C-Eco RIB fragment of the R'plasmid pRPS404 isolated from R.capsulatus［7］.They are cotranscribed in vivo both in R.capsulatus［8］and in Rhodobacter sphaeroides［9］.The gene for the H subunit（puhA）of the RC lies in the BamHI-F fragment of R'［10］，whereas the genes for theβ（pucB）and α（pucA）subunits of LH-Ⅱ are outside this photosynthetic gene cluster［11］.The structural genes for LH-Ⅰ，LH-Ⅱ，RC-L，RC-M，and RC-H in R.capsulatus have been sequenced［7，10，11］.The genes for Bchl and Crt biosynthesis have been mapped by genetic methods［12，13］and transposon mutagenesis［14，15］.
It has been shown that lowering the O2 concentration in cultures of R.capsulatus and R.sphaeroides increases the levels of mRNA for LH-Ⅰ，RC［9，16］，and LH-Ⅱ polypeptides［17］，while the amounts of RNA for pigment-biosynthesis enzymes show only a small increase when the O2 concentration is reduced［13，16］.According to one set of experiments，Crt biosynthesis is not directly regulated by O2［18］.More recently，a sequential and independent expression of LH-Ⅰ and LH-Ⅱ genes has been shown in R.capsulatus during ICM development［19］.
In this paper we report a systematic investigation of mRNAs of photosynthetic genes in R.capsulatus under different growth conditions，using dot blot hybridization as well as blot hybridization of electrophoretically fractionated RNA.The results show a coordinate and differential regulation of the genes for LH（Ⅰ，Ⅱ）and RC（L，M，H）polypeptides and pigment（Bchl，Crt）biosynthesis by light intensity and O2 concentration.
