title: Dual-domain, dual-targeting organellar protein presequences in Arabidopsis can use non-AUG start codons.
abstract: The processes accompanying endosymbiosis have led to a complex network of interorganellar protein traffic that originates from nuclear genes encoding mitochondrial and plastid proteins. A significant proportion of nucleus-encoded organellar proteins are dual targeted, and the process by which a protein acquires the capacity for both mitochondrial and plastid targeting may involve intergenic DNA exchange coupled with the incorporation of sequences residing upstream of the gene. We evaluated targeting and sequence alignment features of two organellar DNA polymerase genes from Arabidopsis thaliana. Within one of these two loci, protein targeting appeared to be plastidic when the 5' untranslated leader region (UTR) was deleted and translation could only initiate at the annotated ATG start codon but dual targeted when the 5' UTR was included. Introduction of stop codons at various sites within the putative UTR demonstrated that this region is translated and influences protein targeting capacity. However, no ATG start codon was found within this upstream, translated region, suggesting that translation initiates at a non-ATG start. We identified a CTG codon that likely accounts for much of this initiation. Investigation of the 5' region of other nucleus-encoded organellar genes suggests that several genes may incorporate upstream sequences to influence targeting capacity. We postulate that a combination of intergenic recombination and some relaxation of constraints on translation initiation has acted in the evolution of protein targeting specificity for those proteins capable of functioning in both plastids and mitochondria.
authors.affiliation: School of Biological Sciences, Beadle Center for Genetics Research, University of Nebraska, Lincoln, NE 68588-0660, USA.
authors.forename: Alan C
authors.initials: AC
authors.lastname: Christensen
authors.forename: Anna
authors.initials: A
authors.lastname: Lyznik
authors.forename: Saleem
authors.initials: S
authors.lastname: Mohammed
authors.forename: Christian G
authors.initials: CG
authors.lastname: Elowsky
authors.forename: Annakaisa
authors.initials: A
authors.lastname: Elo
authors.forename: Ryan
authors.initials: R
authors.lastname: Yule
authors.forename: Sally A
authors.initials: SA
authors.lastname: Mackenzie
mesh_terms: D020121:5' Untranslated Regions / Q000235:genetics; D000595:Amino Acid Sequence / Q000235:genetics; D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence / Q000235:genetics; D018387:Codon, Initiator / Q000235:genetics*; D018388:Codon, Terminator / Q000235:genetics; D004259:DNA-Directed DNA Polymerase / Q000235:genetics / Q000378:metabolism; D039642:Eukaryotic Initiation Factors / Q000235:genetics / Q000378:metabolism*; D018506:Gene Expression Regulation, Plant / Q000235:genetics; D024101:Mitochondrial Proteins / Q000235:genetics / Q000378:metabolism; D015388:Organelles / Q000235:genetics / Q000378:metabolism*; D018087:Plastids / Q000235:genetics / Q000378:metabolism; D014176:Protein Biosynthesis / Q000235:genetics*; D017434:Protein Structure, Tertiary / Q000235:genetics; D021381:Protein Transport / Q000235:genetics; D011995:Recombination, Genetic / Q000235:genetics
chemical_list: D020121:5' Untranslated Regions; D029681:Arabidopsis Proteins; D018387:Codon, Initiator; D018388:Codon, Terminator; D039642:Eukaryotic Initiation Factors; D024101:Mitochondrial Proteins; D004259:DNA-Directed DNA Polymerase