Only little information is available on the function of RpoT polymerases in plants. Heterologously expressed RpoTp, RpoTmp, and RpoTm enzymes of Arabidopsis are active RNA polymerases that prefer circular over linear template DNA. RpoTm and RpoTp (not RpoTmp) exhibit an inherent ability to recognize several mitochondrial and at least one NEP promoter in vitro (Kühn et al. 2007). In mono-cots with two RpoT genes, RpoTm is assumed to represent the catalytic subunit of the mitochondrial RNA polymerase and RpoTp the catalytic subunit of NEP. RpoTp has been detected by specific antibodies in the chloroplasts of rice and maize (Chang et al. 1999; Kusumi et al. 2004) and RpoTm in maize mitochondria (Chang et al. 1999). RpoTp mRNAs are particularly abundant in very young cells of cereal leaves (Chang et al. 1999; Emanuel et al. 2004; Kusumi et al. 2004) in agreement with the proposed importance of NEP activity early in chloroplast development for transcription of the PEP genes (Mullet 1993; see Section 4.1). Expression of RpoTm and RpoTp in monocots is under control of light (Chang et al. 1999) and plastid signal(s) (Emanuel et al. 2004). In Arabidopsis, RpoTm and RpoTmp promoters showed identical expression patterns with highest levels in tissues known for their requirement of high respiration activity (e.g. meristems, tapetum) suggesting a function of both polymerases in mitochondria, whereas
RpoTp expression was highest in green tissues of leaves, stems, and sepals (Emanuel et al. 2006). Like in monocots, transcription of the RpoT genes is stimulated by light in Arabidopsis leaves (T. Preuten, K. Liere, T. Börner, unpublished results), i.e., light-activated expression of phage-type RNA polymerases may be a general phenomenon in angiosperms. Evidence for NEP being represented by RpoTp (probably together with RpoTmp; see below) was provided by studies on transgenic Nicotiana and Arabidopsis plants that overexpressed RpoTp and exhibited an increased usage of certain NEP promoters (Liere et al. 2004). Mutation of the Arabidopsis RpoTp gene led to impaired chloroplast biogenesis and altered accumulation of plastid transcripts (Hricova et al. 2006). Similar observations were made on Arabidopsis plants with reduced RpoTp mRNA levels due to expression of antisense RNA (Emanuel et al., unpublished data). Although the localization of RpoTmp in mitochondria is not in doubt (Kabeya and Sato 2005), its function for this organelle remains obscure so far. RpoTmp was supposed, however, to play a role in plastid gene expression (Baba et al. 2004; Hricova et al. 2006). Arabidopsis lines with impaired RpoTmp function were delayed in chloroplast biogenesis and showed altered plastid transcript levels (Baba et al. 2004). RpoTp/RpoTmp double mutants exhibited a more severe phenotype than both of the single mutants and were extremely retarded in growth (Hricova et al. 2006).
Clearly, more studies are needed to exactly define the function of the different organellar RNA polymerases. First insights into the division of labor between PEP (the bacterial type RNA polymerase) and NEP (probably represented by RpoTp in monocots and RpoTp and RpoTmp in dicots) were obtained from investigations on the use of PEP vs. NEP promoters in different tissues and under the influence of different endogenous and exogenous factors as discussed below.
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