title: TIF1 Represses rDNA replication initiation, but promotes normal S phase progression and chromosome transmission in Tetrahymena.
abstract: The non-ORC protein, TIF1, recognizes sequences in the Tetrahymena thermophila ribosomal DNA (rDNA) minichromosome that are required for origin activation. We show here that TIF1 represses rDNA origin firing, but is required for proper macronuclear S phase progression and division. TIF1 mutants exhibit an elongated macronuclear S phase and diminished rate of DNA replication. Despite this, replication of the rDNA minichromosome initiates precociously. Because rDNA copy number is unaffected in the polyploid macronucleus, mechanisms that prevent reinitiation appear intact. Although mutants exit macronuclear S with a wild-type DNA content, division of the amitotic macronucleus is both delayed and abnormal. Nuclear defects are also observed in the diploid mitotic micronucleus, as TIF1 mutants lose a significant fraction of their micronuclear DNA. Hence, TIF1 is required for the propagation and subsequent transmission of germline chromosomes. The broad phenotypes associated with a TIF1-deficiency suggest that this origin binding protein is required globally for the proper execution and/or monitoring of key chromosomal events during S phase and possibly at later stages of the cell cycle. We propose that micro- and macronuclear defects result from exiting the respective nuclear S phases with physically compromised chromosomes.
authors.affiliation: Department of Medical Biochemistry and Genetics, Texas A&M University System Health Science Center, College Station, TX 77843-1114, USA.
authors.forename: Tara L
authors.initials: TL
authors.lastname: Morrison
authors.forename: J Sebastian
authors.initials: JS
authors.lastname: Yakisich
authors.forename: Donna
authors.initials: D
authors.lastname: Cassidy-Hanley
authors.forename: Geoffrey M
authors.initials: GM
authors.lastname: Kapler
mesh_terms: D000818:Animals; D001665:Binding Sites; D002467:Cell Nucleus; D002875:Chromosomes / Q000235:genetics; D004261:DNA Replication / Q000235:genetics; D016054:DNA, Protozoan / Q000096:biosynthesis / Q000235:genetics*; D004275:DNA, Ribosomal / Q000378:metabolism*; D007700:Kinetics; D048629:Micronuclei, Chromosome-Defective; D008957:Models, Genetic; D009154:Mutation; D009687:Nuclear Proteins / Q000235:genetics* / Q000378:metabolism; D011485:Protein Binding; D012333:RNA, Messenger / Q000378:metabolism; D012093:Replicon; D016196:S Phase; D016808:Tetrahymena thermophila / Q000235:genetics* / Q000378:metabolism; D014157:Transcription Factors / Q000235:genetics* / Q000378:metabolism
chemical_list: D016054:DNA, Protozoan; D004275:DNA, Ribosomal; D009687:Nuclear Proteins; D012333:RNA, Messenger; D014157:Transcription Factors; C093517:transcriptional intermediary factor 1