A third mitochondrial RNA polymerase in the moss Physcomitrella patens

In most organisms, the mitochondrial genes are transcribed by RNA polymerases related to the single-subunit RNA polymerases of bacteriophages like T3 and T7. In flowering plants, duplication(s) of the RpoTm gene coding for the mitochondrial RNA polymerase (RPOTm) led to the evolution of additional RNA polymerases transcribing genes in plastids (RPOTp) or in both mitochondria and plastids (RPOTmp). Two putative RPOTmp enzymes were previously described to be encoded by the nuclear genes RpoTmp1 and RpoTmp2 in the moss Physcomitrella patens. Here, we report on a third Physcomitrella RpoT gene. We determined the sequence of the cDNA. Comparison of the deduced amino acid sequence with sequences of plant organellar RNA polymerases suggests that this gene encodes a functional phage-type RNA polymerase. The 78 N-terminal amino acids of the putative RNA polymerase were fused to GFP and found to target the fusion protein exclusively to mitochondria in Arabidopsis protoplasts. P. patens is the only known organism to possess three mitochondrial RNA polymerases.