33 DNA Replication in Tetrahymena

Abstract

The linear ribosomal RNA gene (rDNA) minichromosome in the somatic macronucleus of Tetrahymena thermophila serves as a paradigm for multiple mechanisms of regulation of chromosomal DNA replication. Whereas chromosomal DNA replication in most eukaryotes is under strict cell cycle control, additional mechanisms appear to be involved in the regulation of rDNA replication in vegetatively growing Tetrahymena cells (Anderson 1972; Engberg et al. 1972; Truett and Gall 1977). All macronuclear chromosomes, including the rDNA minichromosome, segregate randomly and thus appear to lack centromeres. The macronucleus divides by a poorly understood amitotic mechanism, and DNA is often distributed unequally between daughter macronuclei (Doerder 1979). To explain how cells compensate for inequalities that arise from such divisions, it was proposed that a copy number control mechanism is superimposed on cell cycle control of the macronuclear DNA (Preer and Preer 1979; for review, see Larson et al. 1991). In addition, different allelic forms of the rDNA compete with each other for replication in the same macronucleus, and additional copies of the origin region confer a replication advantage to rDNA molecules. These observations are at odds with strict cell cycle control. Therefore, it was concluded that copy number control facilitates the maintenance of rDNA levels in vegetative cells (Larson et al. 1986).

Cell cycle control of DNA replication is also suppressed during developmentally programmed rDNA amplification (Yao et al. 1974). cis-Acting determinants controlling the formation and propagation of the rDNA minichromosome have been identified by classic genetic and DNA transformation approaches (Larson et al. 1986;...