Role of cfactor diversity in transcriptional regulation

To address the question of a specific role of c-factor diversity in transcriptional regulation (see Table 2 summarizing putative roles of c-factors), in vitro reconstitution and transcription experiments using recombinant c-factors and the E. coli core RNA polymerase were carried out by several groups. These again demonstrated that plant c-factor genes encode functional plastidial c-factors (Kestermann et al. 1998; Hakimi et al. 2000; Beardslee et al. 2002; Homann and Link 2003; Privat et al. 2003). While the three mustard c-factors SalSig1, SalSig2, and SalSig3 recognized the psbA promoter, only SalSig1 and SalSig2 recognized the rbcL promoter. However, trnK, trnQ, rps16, and rrn16 (PEP-P1) promoters were rather recognized by SalSig1 and SalSig3, but less efficiently by SalSig2 (Homann and Link 2003). Similar experiments with Arabidopsis c-factors suggested that rather AthSig2 and AthSig3 confer specific recognition of the rbcL and psbA promoters than AthSig1 (Hakimi et al. 2000; Privat et al. 2003). The observed discrepancies in promoter recognition may be due to the heterologous transcription systems with hindered abilities to identify species- and/or PEP-specific regulatory elements at cis- and trans-factor level.

Further efforts to specify distinct functionality of plant c-factors in regulation of plastidial gene expression employed characterization of their expression profiles. Profiling of light-dependent transcription in the red algae Cyanidioschyzon merolae and Cyanidium caldarium revealed light induced accumulation of the mRNAs of c-factor genes (CmeSig1-4; CcaSigB,C; Oikawa et al. 1998; Minoda et al. 2005). Furthermore, CmeSig2 transcript accumulation was additionally increased by high light, indicating that CmeSig2 might be a high-light responsive c-factor (Minoda et al. 2005). Consistent with a prominent role of PEP in leaves, most plastidial c-factor genes of higher plants are expressed in light-dependent manner in green tissue but are silent in non-photosynthetic roots (Isono et al. 1997b; Tanaka et al. 1997; Fujiwara et al. 2000; Oikawa et al. 2000). Moreover, expression of plastidial c-factors seems to be differentially regulated during early Arabidopsis development. AthSig2, AthSig3, AthSig4, and AthSig6 but not AthSig1 and AthSig5 transcripts accumulate in four day old seedlings (Ishizaki et al. 2005), while in eight day old seedlings transcript levels increase for all c-factors (Nagashima et al. 2004a). Additionally, expression of Sig2 transcripts prior to Sig1

Table 2. Roles of a-Facturs in higher plants.

c-tactor

Target

Function

Plant

Gene

Reference

Sigl

general, rbcL

alternative a-factor,

Arabidopsis

Atlg64860

(Hakimi el al. 2000)

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