Six active phage-type RNA polymerase genes in Nicotiana tabacum.
In higher plants, a small nuclear gene family encodes mitochondrial as well as chloroplast RNA polymerases (RNAP) homologous to the bacteriophage T7-enzyme. The Arabidopsis genome contains three such RpoT genes, while in monocotyledonous plants only two copies have been found. Analysis of Nicotiana tabacum, a natural allotetraploid, identified six different RpoT sequences. The study of the progenitor species of tobacco, N. sylvestris and N. tomentosiformis, uncovered that the sequences represent two orthologous sets each of three RpoT genes (RpoT1, RpoT2 and RpoT3). Interestingly, while the organelles are inherited exclusively from the N. sylvestris maternal parent, all six RpoT genes are expressed in N. tabacum. GFP-fusions of Nicotiana RpoT1 revealed mitochondrial targeting properties. Constructs containing the amino-terminus of RpoT2 were imported into mitochondria as well as into plastids. Thus, the dual-targeting feature, first described for Arabidopsis RpoT;2, appears to be conserved among eudicotyledonous plants. Tobacco RpoT3 is targeted to chloroplasts and the RNA is differentially expressed in plants lacking the plastid-encoded RNAP. Remarkably, translation of RpoT3 mRNA has to be initiated at a CUG codon to generate a functional plastid transit peptide. Thus, besides AGAMOUS in Arabidopsis, Nicotiana RpoT3 provides a second example for a non-viral plant mRNA that is exclusively translated from a non-AUG codon.
Hedtke
Boris
B
Institut für Biologie, Humboldt-Universität, Chausseestr. 117, D-10115 Berlin, Germany.
Legen
Julia
J
Weihe
Andreas
A
Herrmann
Reinhold G
RG
Börner
Thomas
T
D017360:Arabidopsis / Q000235:genetics; D001483:Base Sequence; D018076:DNA, Complementary / Q000737:chemistry / Q000235:genetics; D012321:DNA-Directed RNA Polymerases / Q000235:genetics*; D049452:Green Fluorescent Proteins; D008164:Luminescent Proteins / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000235:genetics*; D030821:Plants, Genetically Modified; D011123:Polyploidy; D014176:Protein Biosynthesis; D012317:RNA Phages / Q000201:enzymology / Q000235:genetics; D012333:RNA, Messenger / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D017422:Sequence Analysis, DNA; D012689:Sequence Homology, Nucleic Acid; D014026:Nicotiana / Q000201:enzymology / Q000235:genetics*
D018076:DNA, Complementary; D008164:Luminescent Proteins; D010940:Plant Proteins; D012333:RNA, Messenger; D011993:Recombinant Fusion Proteins; D049452:Green Fluorescent Proteins; C446921:NsRpoT-B protein, Nicotiana sylvestris; D012321:DNA-Directed RNA Polymerases