Regulation of transcription in plastids

Expression of nuclear-encoded plastid-localized gene products is thought to be managed by transcriptional control (Kuhlemeier 1992). While posttranscriptional events contribute significantly to regulation of plastidial gene expression (see Chapters 6, 7; Deng and Gruissem 1987; Stern et al. 1997; Barkan and Gold-

schmidt-Clermont 2000; Monde et al. 2000), transcription of plastid genes was also shown to react to exogenous and endogenous factors such as light and plastid type (Rapp et al. 1992; Mullet 1993; Mayfield et al. 1995; Link 1996).

The circadian rhythm of plastidial gene expression in Chlamydomonas is regulated by transcriptional activity (Salvador et al. 1993; Hwang et al. 1996). Kawa-zoe et al. (2000) could show that the circadian clock-induced transcription is sensitive to cycloheximide, an inhibitor of cytoplasmic translation. However, basal plastidial transcription activity was still maintained. The identity of the cyclo-heximide-sensitive factor(s) needed for circadian peaks of plastidial transcription is still unknown. Expression of the sole c-factor gene CreRpoD (Section 3.2.2; Carter et al. 2004; Bohne et al. 2006) seems also to be under circadian control (Carter et al. 2004). Therefore, a possible dual role of CreRpoD, which might be assisted by topological fluctuations of the plastome (Thompson and Mosig 1990; Salvador et al. 1998), in regulating plastidial gene transcription in Chlamydomonas has been discussed (Misquitta and Herrin 2005).

Transcription activities of most plastid-encoded genes in higher plants increase at an early stage of light-induced plastid development to support rapid construction of the photosynthesis apparatus. Moreover, light-dependent plastid transcription occurs in mature leaves as well as leaves under greening (Greenberg et al. 1989; Schrubar et al. 1990; Baumgartner et al. 1993; DuBell and Mullet 1995; Hoffer and Christopher 1997; Shiina et al. 1998; Satoh et al. 1999; Baena-Gonzalez et al. 2001; Chun et al. 2001; Nakamura et al. 2003). Most prominent examples are photosynthesis-related genes as psbA, psbD-psbC, petG, rbcL, but also housekeeping genes as atpB (Klein et al. 1988; Haley and Bogorad 1990; Klein and Mullet 1990; Sexton et al. 1990; Isono et al. 1997a). Distinctive photo-receptors involved in transcriptional activation of photosynthesis-related genes have been analyzed (Chun et al. 2001; Thum et al. 2001). The developmental stage may influence perception of the light quality. While red light only partially increased plastid transcription, blue light further enhanced overall plastid transcription activity in dark-adapted mature leaves. Therefore, global activation of plastidial transcription after dark adaptation is likely to be mediated by cryptochromes. When exposed to blue light/UV-A an Arabidopsis phyA-mutant displayed lower psbA and rrn16 transcript activities than the wild type suggesting a further role for PhyA in light reception (Chun et al. 2001). Recently, Dhingra et al. (2006) furthermore showed that green light plays a balancing/antagonistic role in controlling gene expression during early photomorphogenic development by downregulating plastidial transcription of genes normally induced by light. As illustrated before, transcriptional response to developmental and environmental changes is likely to involve interaction of the core RNA polymerase with specific regulatory molecules (e.g. c-factors), which may be available only under certain conditions. In silico analyses of nuclear Arabidopsis and rice genes with putative chloroplast transit peptides revealed many putative transcription factors likely to be imported into plastids (Wagner and Pfannschmidt 2006; Schwacke et al. 2007).

Fig. 5. Genes with multiple promoters. Schematic synopsis which shows the multiple PEP promoters of the barley psbD/C operon (HvupsbD), as well as PEP and NEP promoters of the tobacco atpB (NtaatpB), clpP (NtaclpP), and Arabidopsis ycf1 genes (Athycf1). Boxes below the line represent genes on the opposite strand, while open arrowheads denote PEP promoters, filled black arrowheads Type-I NEP promoters, and filled gray arrowheads Type-II NEP promoters. The promoters are named based on their transcription initiation sites in respect to the translation initiation site (+1).

Fig. 5. Genes with multiple promoters. Schematic synopsis which shows the multiple PEP promoters of the barley psbD/C operon (HvupsbD), as well as PEP and NEP promoters of the tobacco atpB (NtaatpB), clpP (NtaclpP), and Arabidopsis ycf1 genes (Athycf1). Boxes below the line represent genes on the opposite strand, while open arrowheads denote PEP promoters, filled black arrowheads Type-I NEP promoters, and filled gray arrowheads Type-II NEP promoters. The promoters are named based on their transcription initiation sites in respect to the translation initiation site (+1).

These factors may represent such candidates to expand the actually known capacity of the chloroplast to regulate its transcription machinery. Additionally, various pathways routing developmental and environmental cues may regulate these factors.

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