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Microbiological Reviews Jun 1994Ciliates contain two types of nuclei: a micronucleus and a macronucleus. The micronucleus serves as the germ line nucleus but does not express its genes. The... (Comparative Study)
Comparative Study Review
Ciliates contain two types of nuclei: a micronucleus and a macronucleus. The micronucleus serves as the germ line nucleus but does not express its genes. The macronucleus provides the nuclear RNA for vegetative growth. Mating cells exchange haploid micronuclei, and a new macronucleus develops from a new diploid micronucleus. The old macronucleus is destroyed. This conversion consists of amplification, elimination, fragmentation, and splicing of DNA sequences on a massive scale. Fragmentation produces subchromosomal molecules in Tetrahymena and Paramecium cells and much smaller, gene-sized molecules in hypotrichous ciliates to which telomere sequences are added. These molecules are then amplified, some to higher copy numbers than others. rDNA is differentially amplified to thousands of copies per macronucleus. Eliminated sequences include transposonlike elements and sequences called internal eliminated sequences that interrupt gene coding regions in the micronuclear genome. Some, perhaps all, of these are excised as circular molecules and destroyed. In at least some hypotrichs, segments of some micronuclear genes are scrambled in a nonfunctional order and are recorded during macronuclear development. Vegetatively growing ciliates appear to possess a mechanism for adjusting copy numbers of individual genes, which corrects gene imbalances resulting from random distribution of DNA molecules during amitosis of the macronucleus. Other distinctive features of ciliate DNA include an altered use of the conventional stop codons.
Topics: Animals; Base Composition; Base Sequence; Cell Nucleus; Chromatin; Chromosomes; Ciliophora; Codon; DNA Replication; DNA Transposable Elements; DNA, Protozoan; Genes, Protozoan; Genetic Code; Molecular Sequence Data; Phylogeny; Tetrahymena
PubMed: 8078435
DOI: 10.1128/mr.58.2.233-267.1994 -
Molekuliarnaia Biologiia 2021A fundamental difference between somatic nuclei (macronuclei) of ciliates and cell nuclei of higher eukaryotes is that the macronuclear genome is a huge number (up to...
A fundamental difference between somatic nuclei (macronuclei) of ciliates and cell nuclei of higher eukaryotes is that the macronuclear genome is a huge number (up to tens or hundreds of thousands) of gene-sized (0.5-25 kb) or subchromosomal (up to 2000 kb) minichromosomes. Electron microscopy shows that macronuclear chromatin usually looks like chromatin bodies or fibrils 200-300 nm thick in the interphase. However, the question of how many DNA molecules are contained in an individual chromatin body remains open. The organization of chromatin in macronuclei was studied in the ciliates Didinium nasutum and three Paramecium sp, which differ in pulsed-field gel electrophoresis (PFGE) karyotype, and compared with the model of topologically associated domains (TADs) of higher eukaryotic nuclei. PFGE showed that the sizes of macronuclear DNAs ranged from 50 to 1700 kb, while the majority of the molecules were less than 500 kb in length. A comparative quantitative analysis of the PFGE and electron microscopic data showed that each chromatin body contained one minichromosome in P. multimicronucleatum in the logarithmic growth phase, while bodies in the D. nasutum macronucleus contained two or more DNA molecules each. Chromatin bodies aggregated during starvation, when activity of the macronuclei decreased, leading to an increase of chromatin body size or the formation of 200- to 300-nm fibrils of several chromatin bodies. A model was proposed to explain the formation of such structures. In terms of topological characteristics, macronuclear chromatin bodies with subchromosomal DNA molecules were found to correspond to higher eukaryotic TADs.
Topics: Cell Nucleus; Chromatin; Chromosomes; Ciliophora; DNA; Macronucleus
PubMed: 34837704
DOI: 10.31857/S0026898421060082 -
Current Science Mar 1947
Topics: Animals; Cell Nucleus; Ciliophora; Eukaryota; Invertebrates; Macronucleus
PubMed: 20243387
DOI: No ID Found -
Genes To Cells : Devoted To Molecular &... Jul 2018Ciliated protozoa possess two morphologically and functionally distinct nuclei: a macronucleus (MAC) and a micronucleus (MIC). The MAC is transcriptionally active and...
Ciliated protozoa possess two morphologically and functionally distinct nuclei: a macronucleus (MAC) and a micronucleus (MIC). The MAC is transcriptionally active and functions in all cellular events. The MIC is transcriptionally inactive during cell growth, but functions in meiotic events to produce progeny nuclei. Thus, these two nuclei must be distinguished by the nuclear proteins required for their distinct functions during cellular events such as cell proliferation and meiosis. To understand the mechanism of the nuclear transport specific to either MAC or MIC, we identified specific nuclear localization signals (NLSs) in two MAC- and MIC-specific nuclear proteins, macronuclear histone H1 and micronuclear linker histone-like protein (Mlh1), respectively. By expressing GFP-fused fragments of these proteins in Tetrahymena thermophila cells, two distinct regions in macronuclear histone H1 protein were assigned as independent MAC-specific NLSs and two distinct regions in Mlh1 protein were assigned as independent MIC-specific NLSs. These NLSs contain several essential lysine residues responsible for the MAC- and MIC-specific nuclear transport, but neither contains any consensus sequence with known monopartite or bipartite NLSs in other model organisms. Our findings contribute to understanding how specific nuclear targeting is achieved to perform distinct nuclear functions in binucleated ciliates.
Topics: Active Transport, Cell Nucleus; Amino Acid Sequence; Animals; Cell Nucleus; Histones; Macronucleus; Micronucleus, Germline; Nuclear Localization Signals; Protein Domains; Protozoan Proteins; Tetrahymena thermophila
PubMed: 29882620
DOI: 10.1111/gtc.12602 -
Methods in Cell Biology 2012Nuclear dualism is a characteristic feature of the ciliated protozoa. Tetrahymena have two different nuclei in each cell. The larger, polyploid, somatic macronucleus...
Nuclear dualism is a characteristic feature of the ciliated protozoa. Tetrahymena have two different nuclei in each cell. The larger, polyploid, somatic macronucleus (MAC) is the site of transcriptional activity in the vegetatively growing cell. The smaller, diploid micronucleus (MIC) is transcriptionally inactive in vegetative cells, but is transcriptionally active in mating cells and responsible for the genetic continuity during sexual reproduction. Although the MICs and MACs develop from mitotic products of a common progenitor and reside in a common cytoplasm, they are different from one another in almost every respect.
Topics: Chromatin; Chromosome Breakage; Chromosomes; DNA Methylation; DNA, Protozoan; Genome, Protozoan; Histones; Macronucleus; Micronucleus, Germline; Mitosis; Protozoan Proteins; Reproduction; Telomere; Tetrahymena; Transcription, Genetic
PubMed: 22444142
DOI: 10.1016/B978-0-12-385967-9.00003-7 -
Cell Motility and the Cytoskeleton May 2000We developed a modified immunofluorescence protocol that permitted visualization of microtubules inside the macronucleus of the ciliate Tetrahymena. Although the...
We developed a modified immunofluorescence protocol that permitted visualization of microtubules inside the macronucleus of the ciliate Tetrahymena. Although the amitotically dividing macronucleus lacks a spindle, an elaborate system of microtubules is assembled inside the macronucleus and between the macronucleus and the cortex. Microtubules could not be detected inside the interphase macronuclei. The early stage of macronuclear division was associated with the assembly of short macronuclear microtubules that localized randomly. The intramacronuclear microtubules were subsequently organized in a radial manner. During elongation of the macronucleus, the distribution of macronuclear microtubules changed from radial to parallel. During constriction of the macronucleus, dense and tangled macronuclear microtubules were detected at the region of nuclear constriction. In the cytosol, microtubules were linking the macronucleus and cell cortex. During recovery after drug-induced depolymerization, microtubules reassembled at multiple foci inside the macronucleus in close proximity to the chromatin. We propose that these microtubules play roles in chromatin partitioning, macronuclear constriction, and positioning of the macronucleus in relation to the cell cortex.
Topics: Animals; Antineoplastic Agents; Antineoplastic Agents, Phytogenic; Benomyl; Cell Division; Cell Nucleus; DNA, Protozoan; Fungicides, Industrial; Interphase; Microscopy, Confocal; Microtubules; Nocodazole; Paclitaxel; Tetrahymena pyriformis
PubMed: 10842330
DOI: 10.1002/(SICI)1097-0169(200005)46:1<17::AID-CM3>3.0.CO;2-C -
Development, Growth & Differentiation Jan 2012Developmentally programmed genome rearrangement has been observed in a variety of eukaryotes from vertebrates to worms to protists, and it provides an interesting... (Review)
Review
Developmentally programmed genome rearrangement has been observed in a variety of eukaryotes from vertebrates to worms to protists, and it provides an interesting exception to the general rule of the constancy of the genome. DNA elimination in the ciliated protozoan Tetrahymena is one of the most well-characterized programmed genome rearrangement events. DNA elimination in the newly formed macronucleus of Tetrahymena is epigenetically regulated by the DNA sequence of the parental macronucleus. Dicer-produced, Piwi-associated small RNAs mediate this epigenetic regulation, probably through a whole-genome comparison of the germline micronucleus to the somatic macronucleus. However, a correlation between small RNAs and programmed genome rearrangement could not be detected in the worm Ascaris suum. Therefore, different types of eukaryotes may have developed unique solutions to perform genome rearrangement.
Topics: Active Transport, Cell Nucleus; Animals; Ascaris suum; DNA; Epigenesis, Genetic; Gene Expression Regulation, Developmental; Gene Rearrangement; Genome; Genome, Protozoan; Heterochromatin; Macronucleus; Micronucleus, Germline; RNA; RNA Interference; Sequence Analysis, DNA; Tetrahymena; Transposases
PubMed: 22103557
DOI: 10.1111/j.1440-169X.2011.01305.x -
The Journal of Eukaryotic Microbiology Sep 2022Ciliates are defined by the presence of dimorphic nuclei as they have both a somatic macronucleus and germline micronucleus within each individual cell. The size and... (Review)
Review
Ciliates are defined by the presence of dimorphic nuclei as they have both a somatic macronucleus and germline micronucleus within each individual cell. The size and structure of both germline micronuclei and somatic macronuclei vary tremendously among ciliates. Except just after conjugation (i.e. the nuclear exchange in their life cycle), the germline micronucleus is transcriptionally inactive and contains canonical chromosomes that will be inherited between generations. In contrast, the transcriptionally active macronucleus contains chromosomes that vary in size in different classes of ciliates, with some lineages having extensively fragmented gene-sized somatic chromosomes while others contain longer multigene chromosomes. Here, we describe the variation in somatic macronuclear architecture in lineages sampled across the ciliate tree of life, specifically focusing on lineages with extensively fragmented chromosomes (e.g. the classes Phyllopharyngea and Spirotrichea). Further, we synthesize information from the literature on the development of ciliate macronuclei, focusing on changes in nuclear architecture throughout life cycles. These data highlight the tremendous diversity among ciliate nuclear cycles, extend our understanding of patterns of genome evolution, and provide insight into different germline and somatic nuclear features (e.g. nuclear structure and development) among eukaryotes.
Topics: Animals; Cell Nucleus; Ciliophora; Life Cycle Stages; Macronucleus
PubMed: 35178799
DOI: 10.1111/jeu.12898 -
Zoological Science Jul 2011The macronucleus of the ciliate Tetrahymena cell contains euchromatin and numerous heterochromatins called chromatin bodies. During cell division, a chromatin aggregate...
The macronucleus of the ciliate Tetrahymena cell contains euchromatin and numerous heterochromatins called chromatin bodies. During cell division, a chromatin aggregate larger than chromatin body appears in the macronucleus. We observed chromatin aggregates in the dividing macronucleus in a living T. thermophila cell, and found that these were globular in morphology and homogeneous in size. To observe globular chromatin clearly, optimal conditions for making it compact were studied. Addition of Mg ion, benomyl and oryzalin, microtubule inhibitors, to cell suspension was effective. Globular chromatin appeared when the micronuclear anaphase began at the cell cortex, and disappeared long after cell separation. Using living cells with a small macronucleus at early log phase, we counted the number of globular chromatin per nucleus and measured the DNA content of globular chromatin in the macronucleus which was stained with Hoechst 33342 by using ImageJ. The number of globular chromatin per nucleus was reduced by half after division, indicating the globular chromatin is a distribution unit of DNA. A globular chromatin contained similar DNA content as that of the macronuclear genome. We developed methods for inducing and isolating a cell with an extremely small macronucleus with a DNA amount of one globular chromatin. These cells grew, divided, and give clones, suggesting that the macronuclear genome is not dispersed within the macronucleus and the globular chromatin may be a macronuclear genome. We named this globular chromatin "macronuclear genome unit" (MGU).
Topics: Animals; Cell Division; DNA; Euchromatin; Heterochromatin; Macronucleus; Tetrahymena
PubMed: 21728796
DOI: 10.2108/zsj.28.482 -
Essays in Biochemistry Sep 2010Ciliated protozoa undergo extensive DNA rearrangements, including DNA elimination, chromosome breakage and DNA unscrambling, when the germline micronucleus produces the... (Review)
Review
Ciliated protozoa undergo extensive DNA rearrangements, including DNA elimination, chromosome breakage and DNA unscrambling, when the germline micronucleus produces the new macronucleus during sexual reproduction. It has long been known that many of these events are epigenetically controlled by DNA sequences of the parental macronuclear genome. Recent studies in some model ciliates have revealed that these epigenetic regulations are mediated by non-coding RNAs. DNA elimination in Paramecium and Tetrahymena is regulated by small RNAs that are produced and operated by an RNAi (RNA interference)-related mechanism. It has been proposed that the small RNAs from the micronuclear genome can be used to identify eliminated DNAs by whole-genome comparison of the parental macronucleus and the micronucleus. In contrast, DNA unscrambling in Oxytricha is guided by long non-coding RNAs that are produced from the somatic parental macronuclear genome. These RNAs are proposed to act as templates for the direct unscrambling events that occur in the developing macronucleus. The possible evolutionary benefits of these RNA-directed epigenetic regulations of DNA rearrangement in ciliates are discussed in the present chapter.
Topics: Animals; DNA; Epigenesis, Genetic; Evolution, Molecular; Genome, Protozoan; RNA
PubMed: 20822488
DOI: 10.1042/bse0480089