Evolution of plant phage-type RNA polymerases: the genome of the basal angiosperm Nuphar advena encodes two mitochondrial and one plastid phage-type RNA polymerases.

BACKGROUND

In mono- and eudicotyledonous plants, a small nuclear gene family (RpoT, RNA polymerase of the T3/T7 type) encodes mitochondrial as well as chloroplast RNA polymerases homologous to the T-odd bacteriophage enzymes. RpoT genes from angiosperms are well characterized, whereas data from deeper branching plant species are limited to the moss Physcomitrella and the spikemoss Selaginella. To further elucidate the molecular evolution of the RpoT polymerases in the plant kingdom and to get more insight into the potential importance of having more than one phage-type RNA polymerase (RNAP) available, we searched for the respective genes in the basal angiosperm Nuphar advena.

RESULTS

By screening a set of BAC library filters, three RpoT genes were identified. Both genomic gene sequences and full-length cDNAs were determined. The NaRpoT mRNAs specify putative polypeptides of 996, 990 and 985 amino acids, respectively. All three genes comprise 19 exons and 18 introns, conserved in their positions with those known from RpoT genes of other land plants. The encoded proteins show a high degree of conservation at the amino acid sequence level, including all functional crucial regions and residues known from the phage T7 RNAP. The N-terminal transit peptides of two of the encoded polymerases, NaRpoTm1 and NaRpoTm2, conferred targeting of green fluorescent protein (GFP) exclusively to mitochondria, whereas the third polymerase, NaRpoTp, was targeted to chloroplasts. Remarkably, translation of NaRpoTp mRNA has to be initiated at a CUG codon to generate a functional plastid transit peptide. Thus, besides AGAMOUS in Arabidopsis and the Nicotiana RpoTp gene, N. advena RpoTp provides another example for a plant mRNA that is exclusively translated from a non-AUG codon. In contrast to the RpoT of the lycophyte Selaginella and those of the moss Physcomitrella, which are according to phylogenetic analyses in sister positions to all other phage-type polymerases of angiosperms, the Nuphar RpoTs clustered with the well separated clades of mitochondrial (NaRpoTm1 and NaRpoTm2) and plastid (NaRpoTp) polymerases.

CONCLUSIONS

Nuphar advena encodes two mitochondrial and one plastid phage-type RNAP. Identification of a plastid-localized phage-type RNAP in this basal angiosperm, orthologous to all other RpoTp enzymes of flowering plants, suggests that the duplication event giving rise to a nuclear gene-encoded plastid RNA polymerase, not present in lycopods, took place after the split of lycopods from all other tracheophytes. A dual-targeted mitochondrial and plastididal RNA polymerase (RpoTmp), as present in eudicots but not monocots, was not detected in Nuphar suggesting that its occurrence is an evolutionary novelty of eudicotyledonous plants like Arabidopsis.