Info

PEP denotes PEP promoters, NEP represents NEP Type-1 promoters, NEP 0 indicates NEP Type-il promoters, and NEP Pc denotes Pe promoters; nd. indicates not yet identified promoters.

PEP denotes PEP promoters, NEP represents NEP Type-1 promoters, NEP 0 indicates NEP Type-il promoters, and NEP Pc denotes Pe promoters; nd. indicates not yet identified promoters.

gene is transcribed from at least three NEP (PatpB-255, -502/-488, -611) and two PEP promoters (Fig. 5; PatpB-289, -329; Hajdukiewicz et al. 1997), but only one PEP and one NEP promoter are driving this gene in Arabidopsis (PEP-PatpB-520, NEP-PatpB-318; Swiatecka-Hagenbruch et al. 2007) and maize (NEP-PatpB-601, PEP-PatpB-298; Silhavy and Maliga 1998a). In case of clpP, the tobacco gene has two Type-I NEP (PclpP-173, -511), one PEP (PclpP-95) and the main Type-II NEP initiation sites (Fig. 5; PclpP-53; Hajdukiewicz et al. 1997; Sriraman et al. 1998a). The Arabidopsis clpP gene has a PEP (PclpP-115) and a Type-II NEP initiation site (PclpP-58; Sriraman et al. 1998a; Swiatecka-Hagenbruch et al. 2007). The maize gene, however, is transcribed from a sole Type-I NEP promoter (PclpP-111; Silhavy and Maliga 1998a) indicating a high diversity in promoter usage in different species (see Table 1 for a comparison of promoters of more plastidial genes in different plants). Furthermore, an increasing number of genes are reported to be co-transcribed with other genes within an operon and to additionally possess an individual promoter upstream of their coding region (e.g. trnG and psbA; Meng et al. 1991; Nickelsen and Link 1991; Kapoor et al. 1994; Liere and Link 1994; Liere et al. 1995).

The rrn16 promoters are an interesting and well investigated example of the diversity of promoter usage within a highly conserved DNA sequence even in closely related species. The main rrn operon promoter in tobacco is a CT70-type PEP promoter (P1 or Nt-Prrn-114; Vera and Sugiura 1995; Allison et al. 1996). In barley, maize, and pea the rrn operon is also transcribed from the P1 CT70-type PEP promoter (Fig. 6; Strittmatter et al. 1985; Sun et al. 1989; Hübschmann and Börner 1998). Additionally, the rrn operon in tobacco has a NEP promoter (Fig. 6; P2 or Nta-Prrn-62), inactive in chloroplasts, but functional in BY2 tissue culture cells and in plants lacking PEP (Vera and Sugiura 1995; Allison et al. 1996). Conversely, there is no active NEP promoter directly upstream of the rrn operon in maize plastids (Silhavy and Maliga 1998a). In spinach chloroplasts transcription of the rrn operon initiates within a region between the promoter elements of P1 (Fig. 4, 6; Pc promoter; Baeza et al. 1991; Iratni et al. 1994, 1997). However, the CT70-type promoter sequences are not utilized in vivo. Interestingly, the Pc site appears to be faithfully recognized by partially purified mustard PEP in vitro (Pfannschmidt and Link 1997). A good candidate for the Pc activating factor in spinach is CDF2 (see Section 4.2.1; Iratni et al. 1994, 1997; Bligny et al. 2000).

In Arabidopsis, rrn operon transcripts were mapped to both the major PEP P1 and the spinach Pc initiation sites (Fig. 6; Sriraman et al. 1998b; Swiatecka-Hagenbruch et al. 2007). A study of rrn promoters in heterologous plastids indicates that tobacco plastids lack the factor required for transcription from Pc, while spinach has an intact P1 promoter but lacks the cognate P1 activator (Sriraman et al. 1998b). However, in tobacco an rRNA operon upstream activator region (RUA) that is conserved in monocot and dicot species has been identified (Fig. 4; Suzuki et al. 2003). It has been suggested that the -10 element plays only a limited role in rrn16 P1 recognition and that CT-factor interaction is replaced in part by direct PEP-RUA (protein-DNA) interaction or by protein-protein interaction between the PEP and a putative RUA-binding transcription factor.

Spinach trnV Arabidopsis trnV Tobacco trnV Barley trnV

Fig. 6. Diverse promoters of the rrn operon in spinach, Arabidopsis, tobacco, and barley. The distinct promoters that are used in different species are shown by the schematic representation of the transcription initiation sites between trnV and rrn16 (marked by arrows). P1 and P2 mark transcription initiation sites by PEP (open circle) and NEP (filled circle). Transcript initiation in spinach and Arabidopsis from a yet uncharacterized NEP promoter is indicated by Pc. A dashed vertical line indicates an RNA processing site.

Interestingly, ycfl in Arabidopsis is transcribed from a strongly conserved NEP promoter as in tobacco (NEP-AthPyc/1-39; Swiatecka-Hagenbruch et al. 2007). However, a PEP promoter located at the NEP promoter position takes over transcription in green leaves (PEP-AthPy/1-34). With the rrn16 Pc and PEP promoters in Arabidopsis, this is a rare incident where a defined DNA sequence serves as a promoter for both NEP and PEP.

The role of most multiple promoters upstream of plastidial genes and operons is not fully understood, however, some are well characterized. The blue-light-responsive promoter (BRLP) amongst the three PEP promoters of the psbD-psbC operon, for example, is thought to differentially maintain the ability to re-synthesize and replace damaged D2 and CP43 photosystem components in mature chloroplasts.

In spite of the observed diversity of plastidial promoter usage between different species of higher plants, the data support also the existence of common themes in promoter usage that have been deduced mainly from studies on transcription in tobacco plastids. Mixed NEP and PEP promoters typically are found upstream of housekeeping genes which need to be transcribed during full plastidial development (Maliga 1998). Consequently, both promoter types are believed to differentially express their cognate gene during plant development (reviewed in Liere and Maliga 2001). NEP promoters are generally recognized in youngest and non-green tissues early in plant development, while PEP takes over in maturating, photosyn-thetically active chloroplasts (Bisanz-Seyer et al. 1989; Baumgartner et al. 1993; Hajdukiewicz et al. 1997; Kapoor et al. 1997; Emanuel et al. 2004).

This simple model has been challenged by results from transcriptional reanalyses of tobacco Arpo mutants lacking PEP (Krause et al. 2000; Legen et al. 2002). Large spurious transcripts initiated by NEP cover the entire plastome in these mutants, suggesting that besides selective promoter utilization, posttranscrip-tional processes also determine the transcript pattern of plastids. Furthermore, it has been shown in maize, that although the NEP enzyme becomes less abundant

Fig. 6. Diverse promoters of the rrn operon in spinach, Arabidopsis, tobacco, and barley. The distinct promoters that are used in different species are shown by the schematic representation of the transcription initiation sites between trnV and rrn16 (marked by arrows). P1 and P2 mark transcription initiation sites by PEP (open circle) and NEP (filled circle). Transcript initiation in spinach and Arabidopsis from a yet uncharacterized NEP promoter is indicated by Pc. A dashed vertical line indicates an RNA processing site.

as chloroplasts mature its transcriptional activity increases (Cahoon et al. 2004). The stability of the RNA generated by NEP, however, declines during chloroplast development. For transcripts generated by PEP, transcription rates increase as chloroplasts develop, whereas RNA stability remains constant or increases. Hence, in a proposed model for maize plastidial biogenesis, NEP-controlled transcript accumulation changes little during plastidial development while PEP-controlled transcript accumulation increases (Cahoon et al. 2004). In other species, a strong correlation between the transcribing enzyme (NEP or PEP) and the pattern of transcript accumulation was not observed (Zoschke et al. 2007).

Since genes exclusively transcribed by NEP encode housekeeping functions like the rpoB gene/operon and rps15, NEP should be still necessary for proper gene expression and regulation also in mature chloroplasts. Furthermore, an additional role of NEP as an SOS-enzyme in plastidial transcription has been proposed by Schweer et al. (2006). Analyses of transcript accumulation of atpB in an Ath-Sig6 knockout mutant suggested that a further upstream located NEP promoter compensates for failing transcription from the main PEP promoter. Indeed, NEP and PEP are active throughout leaf development in Arabidopsis, although PEP plays a major role in mature leaves (Cahoon et al. 2004; Zoschke et al. 2007). Interestingly, exclusively PEP-transcribed genes code for proteins with a role in photosynthesis. As the major active polymerase in mature chloroplasts, present data point to PEP as a prominent target for regulation signals including redox control, not yet determined for NEP (for review see Forsberg et al. 2001; Liere and Maliga 2001; Pfannschmidt and Liere 2005). Since plants that turn to a parasitic lifestyle lost photosynthetic genes as well as PEP promoters (Wolfe et al. 1992a; Wolfe et al. 1992b; Krause et al. 2003; Berg et al. 2004), transcription and regulation of plastidial gene expression by PEP might be connected to photosynthesis.

Was this article helpful?

0 0

Post a comment