title: Evolutionary aspects of a unique internal mitochondrial targeting signal in nuclear-migrated rps19 of sugar beet (Beta vulgaris L.).
abstract: The endosymbiotic theory postulates that many genes migrated from endosymbionts to the nuclear genomes of their hosts. Some migrated genes lack presequences directing proteins to mitochondria, and their mitochondrial targeting signals appear to be inscribed in the core coding regions as internal targeting signals (ITSs). ITSs may have evolved after sequence transfer to nuclei or ITSs may have pre-existed before sequence transfer. Here, we report the molecular cloning of a sugar beet gene for ribosomal protein S19 (Rps19; the first letter is capitalized when the gene is a nuclear gene). We show that sugar beet Rps19 (BvRps19) is an ITS-type gene. Based on amino-acid sequence comparison, dicotyledonous rps19s (the first letter is lower-cased when the gene is a mitochondrial gene), such as tobacco rps19 (Ntrps19), resemble an ancestral form of BvRps19. We investigated whether differences in amino-acid sequences between BvRps19 and Ntrps19 were involved in ITS evolution. Analyses of the intracellular localization of chimaeric GFP-fusion proteins that were transiently expressed in Welsh onion cells showed that Ntrps19-gfp was not localized in mitochondria. When several BvRps19-type amino acid substitutions, none of which was seen in any other angiosperm rps19, were introduced into Ntrps19-gfp, the modified Ntrps19-gfp became localized in mitochondria, supporting the notion that an ITS in BvRps19 evolved following sequence transfer to nuclei. Not all of these substitutions were seen in other ITS-type Rps19s, suggesting that the ITSs of Rps19 are diverse.
authors.affiliation: Laboratory of Genetic Engineering, Research Faculty of Agriculture, Hokkaido University, N-9, W-9, Kita-ku, Sapporo 060-8589, Japan.
authors.forename: Muneyuki
authors.initials: M
authors.lastname: Matsunaga
authors.forename: Yoshiya
authors.initials: Y
authors.lastname: Takahashi
authors.forename: Rika
authors.initials: R
authors.lastname: Yui-Kurino
authors.forename: Tetsuo
authors.initials: T
authors.lastname: Mikami
authors.forename: Tomohiko
authors.initials: T
authors.lastname: Kubo
mesh_terms: D000595:Amino Acid Sequence; D027461:Beta vulgaris / Q000378:metabolism*; D005075:Biological Evolution; D002467:Cell Nucleus / Q000235:genetics*; D003001:Cloning, Molecular; D004272:DNA, Mitochondrial / Q000235:genetics; D018745:Genome, Plant; D008928:Mitochondria / Q000235:genetics / Q000378:metabolism*; D051336:Mitochondrial Membranes / Q000378:metabolism; D008969:Molecular Sequence Data; D010802:Phylogeny; D012269:Ribosomal Proteins / Q000235:genetics / Q000378:metabolism*; D017386:Sequence Homology, Amino Acid; D014026:Nicotiana / Q000378:metabolism*
chemical_list: D004272:DNA, Mitochondrial; D012269:Ribosomal Proteins; C040343:ribosomal protein S19