Abstract
Transcription of mitochondrial genes in animals, fungi, and plants relies on the activity of T3/T7 phage-type RNA polymerases. Two such enzymes, RPOTm and RPOTmp, are present in the mitochondria of eudicotyledonous plants; RPOTmp is additionally found in plastids. We have characterized the transcriptional role of the dual-targeted RNA polymerase in mitochondria of Arabidopsis thaliana. Examination of mitochondrial transcripts in rpoTmp mutants revealed major differences in transcript abundances between wild-type and rpoTmp plants. Decreased levels of specific transcripts were correlated with reduced abundances of the respiratory chain complexes I and IV. Altered transcript levels in rpoTmp were found to result from gene-specific transcriptional changes, establishing that RPOTmp functions in distinct transcriptional processes within mitochondria. Decreased transcription of specific genes in rpoTmp was not associated with changes in promoter utilization; therefore, RPOTmp function is not promoter specific but gene specific. This implies that additional gene-specific elements direct the transcription of a subset of mitochondrial genes by RPOTmp.
Highlights
The evolution of mitochondria from the bacterial endosymbiont was accompanied by a loss of the bacterial-type RNA polymerase and replacement by a T3/T7 phage-type RNA polymerase in a common ancestor of almost all extant eukaryotes
We found transmission of rpoTm alleles by both male and female gametes to be reduced by two-thirds, which could be due to decreased rpoTm gamete fitness, such as partial gamete lethality, or decreased fitness of heterozygous embryos
These T-DNA insertion lines were used previously to study Arabidopsis plants lacking RPOTmp (Baba et al, 2004; Courtois et al, 2007), but no molecular defects were detected in mitochondria
Summary
The evolution of mitochondria from the bacterial endosymbiont was accompanied by a loss of the bacterial-type RNA polymerase and replacement by a T3/T7 phage-type RNA polymerase in a common ancestor of almost all extant eukaryotes (reviewed in Tracy and Stern, 1995; Gray and Lang, 1998; Hess and Borner, 1999). Based on a developmental analysis of RPOT transcript levels and on the observation that in the mutant, the induction of several plastid genes in dark-grown seedlings upon illumination was delayed, Baba et al (2004) proposed RPOTmp to be the key RNA polymerase transcribing organellar genes during early seedling development and favored a role of both RPOTm and RPOTp at a later developmental stage. RPOTmp and RPOTp were suggested to have partially redundant roles in plastids as plants lacking either RPOTmp or RPOTp showed similar morphological and developmental alterations when compared with the wild type; a double mutant disrupted in both of these enzymes was seedling lethal (Hricova et al, 2006)
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