SYNOPSIS

The macro- and micronucleus of Tetrahymena pyriformis are formed from a common diploid synkaryon during conjugation. Shortly after the 2nd postzygotic division, distinct morphologic and physiologic differences develop between the 2 nuclei. Micronuclei remain small, presumably diploid, and electronmicroscopic observations indicate that micronuclear DNA is contained in a dense, fibrous, chromosome-like coil. Macronuclei contain considerably more DNA than micronuclei, and the DNA of the macronucleus is found largely in the chromatin bodies typical of ciliate nuclei. The functional differences between macro- and micronuclei in vegetative cells also are striking. The template activity of DNA in the micronucleus is highly restricted compared to that in the macronucleus. Micronuclei synthesize and contain little RNA, and do not contain either nucleoli or ribonucleoprotein granules. Macronuclei, on the other hand, synthesize and contain large amounts of RNA and have many nucleoli and ribonucleoprotein granules. Macro- and micronuclei also have distinct differences in the timing of DNA synthesis during the cell cycle and in the timing and mechanism of nuclear division. Finally, during conjugation the macronucleus becomes pycnotic and disappears while the micronucleus undergoes meiosis and fertilization, ultimately giving rise to new macro- and new micronuclei. In short, the macro- and micronuclei of Tetrahymena provide an excellent system for studying the molecular mechanisms by which the same (or related) genetic information is maintained in different structural and functional states.



Methods have been devised to isolate and purify macro- and micronuclei of Tetrahymena in the hope of correlating differences in the nucleoprotein composition of these nuclei with differences in their structure and function. The DNAs of macro- and micronuclei have been found to differ markedly in their content of a methylated base, N6-methyl adenine, and major differences in the histones of the 2 nuclei have been observed. Macronuclei contain histones similar to those found in vertebrate nuclei, while 2 major histone fractions seem to be missing in micronuclei. In addition, histone fraction F2A1 which is found in multiple, acetylated forms in macronuclei, is present only as a single, unacetylated form in micronuclei.

Macro‐ and Micronuclei of Tetrahymena pyriformis: A Model System for Studying the Structure and Function of Eukaryotic Nuclei*†

INTRODUCfION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . 204 Genome Constancy and Variability in Development .. . . . . . . . . . ......... ...... . ... .... . . 204 Why Study Tetrahymena DNA? 205 Macroand micronuclei .. . . . . . .. . . . . ..... ... . . . . . . . . . . . . .. .. . .. . ..... . .. . . . . . . .. . .. . .. . . . .. . . .. .... . . . . ... . .. . . .. . . . 205 The organism . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . ....... 206 Phenotypic assortment . . . .. . ..... ..... ....... . ... . .. ... 207 BULK PROPERTIES OF MACRO-AND MICRONUCLEAR DNA . . . . . . . . . . . . . . . . . . . . 208 DNA Content ....... ....... . . ... . . . . . . . 208 Buoyant Density and Thermal Denaturation 209 Modified Bases . . . . . . . .. . .. . .. . .. . .. 210 Sequence Interspersion in Macronuclei 210 Kinetic Complexity 01 Macronuclear DNA 212 Comparison 01 Macroand Micronuclear DNA Sequences 213 Molecular Weight 215 SPECIFIC DNA SEQUENCES . . ... . . . . ..... . ... . .... ... ..... ... . .. ... . . . . . . . . . . . . .... ... ..... ... ... . . .. . .... . . .. . 217 The Genes lor 26S + 17S Ribosomal RNA (rRNA) 217 Structure of macronuclear ribosomal genes 217 Number of macronuclear ribosomal genes 219 The macronuclear ribosomal gene transcription unit .. . . . . . 220 An intervening sequence in macronuclear rDNA . . . . . . . . . . . . . ... . . . .. . . . .. ...... . .. . . . ..... . 221 Replication of macronuclear rDNA 222 Homogeneity of macronuclear rDNA 223 rDNA amplification .. , 224 5S rRNA Genes .i 228 C�1 Sequences 230 Cloned DNA Sequences . . . . .. . . . ... . . . . . . . . . . . . . . . . . . .. . . .. . . . . 231 SUMMARY AND PROSPECfS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . .. .. . .. . . .. . . . .. . .. . .. ...... 232

Genome organization and reorganization in tetrahymena

1 – Localization of Nuclear Functions as Revealed by Ultrastructural Autoradiography and Cytochemistry

Publisher Summary Tetrahymena is an extraordinarily tractable biological system for molecular and genetic analysis. Many genes that are repeated in the genomes of multicellular eukaryotes exist in only one or two copies in the Tetrahymena genome. The two outstanding examples of this principle are the rDNA and the histones. The presence of single-copy rDNA in the germline makes the system uniquely suited for genetic analysis of rDNA replication. Similarly, the organization of the histone genes permits their disruption. The support of cellular viability by the macronucleus allows for the disruption of micronuclear functions in ways that would be cell lethal in other eukaryotic systems. Most notably, mutations in mitotic function can be analyzed in cells where continued cell growth is supported by the amitotically dividing macronucleus. The unusual ability to manipulate the germline independent of the soma provides significant technical advantages for genetic analysis. Strains containing deleterious but nonlethal gene alleles can be readily constructed and propagated. Drug-resistant heterokaryons can be used to select for sexual progeny. Whole genome homozygotes can be recovered in a single mating, which greatly facilitates the isolation of recessive mutations.

Renée Charret

Papers

l'ADN nucléolaire chez Tetrahymena pyriformis: Chronologie de sa réplication