title: Amplification of tandemly repeated origin control sequences confers a replication advantage on rDNA replicons in Tetrahymena thermophila.
abstract: The macronuclear rRNA genes (rDNA) in the ciliate Tetrahymena thermophila are normally palindromic linear replicons, containing two copies of the replication origin region in inverted orientation. A circular plasmid containing a single Tetrahymena rRNA gene (one half palindrome) joined to a tandem repeat of a 1.9-kilobase (kb) rDNA segment encompassing the rDNA replication origin and known replication control elements was used to transform Tetrahymena macronuclei by microinjection. This plasmid was shown previously to have a replication advantage over the rDNA allele of the recipient cell strain (G.-L. Yu and E. H. Blackburn, Proc. Natl. Acad. Sci. USA 86:8487-8491, 1990). During vegetative cell divisions, the circular and palindromic rDNAs were rapidly replaced by novel, successively longer linear rDNAs that eventually contained up to 30 tandem 1.9-kb repeats, resulting from homologous but unequal crossovers between the 1.9-kb repeats. We present evidence to show that increasing the number of copies of the replication control regions increases the replicative advantage of the rDNA, the first such situation for a cellular nuclear replicon in a eucaryote.
authors.affiliation: Department of Molecular and Cell Biology, University of California, Berkeley 94720.
authors.forename: G L
authors.initials: GL
authors.lastname: Yu
authors.forename: E H
authors.initials: EH
authors.lastname: Blackburn
mesh_terms: D000818:Animals; D015139:Blotting, Southern; D004275:DNA, Ribosomal / Q000235:genetics*; D005784:Gene Amplification; D010957:Plasmids; D011995:Recombination, Genetic; D012045:Regulatory Sequences, Nucleic Acid; D012091:Repetitive Sequences, Nucleic Acid; D012093:Replicon; D015183:Restriction Mapping; D013768:Tetrahymena / Q000235:genetics*
chemical_list: D004275:DNA, Ribosomal