Plastids divide in a similar manner as bacteria. Each plastid in a plant contains identical circular copies of the plastid chromosome, the plastome. In addition to monomeric circles, dimers, trimers, and tetramers exist, but also numerous linear and even more complex molecules of different sizes. The number of plastids per cell and of plastomes per plastid changes species-specifically from cell-type to cell-type and may vary during the development of plants (Butterfass 1980; Herrmann and Possingham 1980; Lopez-Juez and Pyke 2005; see Chapters 2, 3, 4). Adjusting the copy number of plastomes per cell could be a way to respond to different needs for plastid gene products, in particular of rRNAs as Bendich (1987) suggested. The striking increase of plastome copies at the beginning of the development of chloroplasts from proplastids is certainly a precondition for the biogenesis of the photosynthetic apparatus in young leaf cells. Other ways to control gene expression at the DNA level could be via alteration of the DNA conformation (Stirdivant et al. 1985; Gauly and Kossel 1989; Sekine et al. 2002) or differential methylation (Ngernprasirtsiri et al. 1989; Kobayashi et al. 1990; Ngernprasirtsiri and Akazawa 1990). Both ways are investigated in only a few
Topics in Current Genetics, Vol. 19 R. Bock (Ed.): Cell and Molecular Biology of Plastids DOI 10.1007/4735_2007_0232 / Published online: 13 June 2007 © Springer-Verlag Berlin Heidelberg 2007
studies and at least the latter one may be an exception rather than the rule (Hess et al. 1993; Isono et al. 1997a).
Early studies on gene expression in chloroplasts revealed specific effects of light on the expression of the psbA gene and of the cell type (mesophyll vs. bundle sheath cells) on the expression of rbcL (Bedbrook et al. 1978; Link et al. 1978) suggesting an important role of differential transcription like in bacteria. Further studies during the 1980's, however, revealed important contributions of posttrans-criptional processes in controlling the levels of gene products (plastid RNAs and proteins) and only a minor role for transcription in the regulation of gene expression in plastids during plant development (Deng and Gruissem 1987; Gruissem et al. 1988). While the importance of posttranscriptional processes in the control of RNA and protein levels remained undisputed until today (see Chapters 6-10), the view on the role of transcription changed again during the 1990's with the discovery of differential transcription of house-keeping vs. photosynthesis genes, of light-induced differential transcription of several genes, and of additional promoters within operons (Mullet 1993). Moreover, the machinery for transcription in plastids of angiosperms unexpectedly turned out to be more complex as known from bacteria and to need different RNA polymerases, although plastids possess a much smaller genome than their cyanobacterial ancestors (Stern et al. 1997; Gray and Lang 1998; Hess and Börner 1999; Liere and Maliga 2001). This chapter describes the components of the transcriptional apparatus in plastids and their roles in transcription and its regulation.
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