The RNA Polymerase

RNA polymerases encoded by mitochondrial linear plasmids from both plants and fungi are of the bacteriophage T7 type (Kuzmin et al. 1988; Oeser 1988; Oeser and Tudzynski 1989; Kempken et al. 1992; Cermakian et al. 1997). The latter does not employ a protein-primed replication mode; indeed, it is only distantly related to protein-primed replicating bacteriophages. The T7 RNA polymerase constitutes an archetypical single subunit enzyme which is not only encoded by T7-like phages and linear plasmids, but also is the common RNA polymerase in (plasmid free) mitochondria throughout the eu-karya (Masters et al. 1987; Tiranti et al. 1997; Weihe et al. 1997; Cermakian et al. 1996). It has been suggested that the mitochondrial RNA polymerase, acquired from a T7 progenitor early in eukaryotic evolution, replaced the eubacterial multisubunit RNA polymerase that was originally present in the bacterial progenitor of the mitochondrium (Shutt and Gray 2006). Thus, while the DNA polymerase encoded by mitochondrial linear plasmids is clearly of viral origin (similar to adenoviral and phi29-like enzymes), it remains obscure whether the RNA polymerase encoding gene was acquired directly from a viral progenitor or from the eukaryotic host. Interestingly, phi29-like phages do not possess their own RNA polymerases but recruit host encoded enzymes (reviewed by Meijer et al. 2001). It remains to be elucidated why T7-like RNA polymerases are required for maintenance of linear plasmids residing in mitochondria, although there is a T7-like polymerase anyway.

Though T7-like RNA polymerase genes are routinely found in known mi-tochondrial linear plasmids, it is not to be excluded that plasmid systems devoid of such genes may exist. Mitochondrial linear plasmids are known which encode a TP-DNA polymerase only (e.g., pAI1 and pMC2-3); however, sequencing of the entire set of plasmids present in such strains is necessary to resolve whether there is a T7-like RNA polymerase gene.

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