Submitted to: Photosynthesis Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 9, 1995
Publication Date: N/A
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Interpretive Summary: The photosynthetic machinery of all higher plant cells is located within the chloroplast. However, the genetic code for the proteins of the photosynthetic machinery is not entirely located in the chloroplast; many photosynthesis genes are localized on nuclear chromosomes. Hence, the assembly of the photosynthetic apparatus requires the coordinated expression of genes located in both the nucleus and in the chloroplast. Understanding the signals that turn on photosynthesis genes in both the chloroplast and nucleus may ultimately lead to an increase in the fixation of atmospheric carbon dioxide by plants and thus greater crop productivity. We examined the biology of a photosynthetic-deficient mutant of tobacco to better understand the signals in the plant that trigger the assembly of the photosynthetic machinery. This information is critical to on- going scientific studies that are attempting to increase plant fixation of atmospheric carbon dioxide, which has been elevated by the combustion of fossil fuels.
Technical Abstract: Expression of the genes for ribulose-1,5-bisphosphate carboxylase/oxygenase Rubisco (I), Rubisco activase (II) and ribulose-5-phosphate kinase (III) and accumulation of the polypeptides were examined in chlorophyllous and chlorotic sectors of the DP1 mutant of tobacco. Plastids from chlorotic sectors of this variegated plastome mutant contained 30S and 50S ribosomal subunits, but had abnormally low levels of plastid polysomes. Consequently, mutant plastids were translationally repressed, unable to synthesize plastid-encoded polypeptides including the large subunit of I despite the presence of the corresponding mRNAs. Transcripts of I small subunit (SS) accumulated to near wild type levels in chlorotic sectors, but there was little accumulation of the I small subunit polypeptide or holoenzyme. Messenger-RNA isolated from chlorotic sectors effectively directed the synthesis of I SS in vitro, suggesting that posttranslational factors were responsible for the decrease in I SS abundance. Transcripts of II and III also accumulated to near wild type levels in chlorotic sectors and a diurnal rhythm in the abundance of II mRNA was detected in green and chlorotic sectors. Despite the low abundance of Rubisco holoenzyme in chlorotic sectors, polypeptides II and III accumulated to significant levels. The data indicate that the developmental events governing the accumulation of II and III and the diurnal regulation of the expression of II were not dependent on the attainment of photosynthetically competent plastids or the accumulation of I.