Rate of phenotypic assortment in Tetrahymena thermophila

Phosphorylation of linker histone H1 regulates gene expression in vivo by creating a charge patch

In Tetrahymena, histone H1 phosphorylation can regulate transcription and mimics loss of H1 from chromatin. We investigated the mechanism by which H1 phosphorylation affects transcription. Tetrahymena strains were created containing mutations in H1 that mimicked the charge of the phosphorylated region without mimicking the structure or increased hydrophilicity of the phosphorylated residues. Whenever the charge resembled that of the phosphorylated state, the induced expression of the CyP1 gene was greatly inhibited. Whenever the charge was similar to that of the dephosphorylated state, the CyP1 gene was induced normally. These results argue strongly that phosphorylation of H1 acts by changing the overall charge of a small domain, not by phosphate recognition or by creating a site-specific charge.

Autonomously replicating macronuclear DNA pieces are the physical basis of genetic coassortment groups in Tetrahymena thermophila

The macronucleus of the ciliate Tetrahymena thermophila contains a fragmented somatic genome consisting of several hundred identifiable chromosome pieces. These pieces are generated by site-specific fragmentation of the germline chromosomes and most of them are represented at an average of 45 copies per macronucleus. In the course of successive divisions of an initially heterozygous macronucleus, the random distribution of alleles of loci carried on these copies eventually generates macronuclei that are pure for one allele or the other. This phenomenon is called phenotypic assortment. We have previously reported the existence of loci that assort together (coassort) and hypothesized that these loci reside on the same macronuclear piece. The work reported here provides new, rigorous genetic support for the hypothesis that macronuclear autonomously replicating chromosome pieces are the physical basis of coassortment groups. Thus, coassortment allows the mapping of the somatic genome by purely genetic means. The data also strongly suggest that the random distribution of alleles in the Tetrahymena macronucleus is due to the random distribution of the MAC chromosome pieces that carry them.

Reorganization of microtubules in the amitotically dividing macronucleus of tetrahymena

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.

Tetrahymena macronuclear genome mapping: colinearity Of macronuclear coassortment groups and the micronuclear map on chromosome 1l

The genetics of the ciliate Tetrahymena thermophila are richer than for most other eukaryotic cells, because Tetrahymena possesses two genomes: a germline (micronuclear) genome that follows a Mendelian model of genetic transmission and a somatic (macronuclear) genome, derived from the micronuclear genome by fragmentation, which follows a different genetic transmission model called phenotypic assortment. While genetic markers in the micronucleus fall into classical linkage groups under meiotic recombination and segregation, the same markers in the macronucleus fall into coassortment groups (CAGs) under phenotypic assortment by the random distribution of MAC chromosome pieces. We set out to determine whether genomic mapping in the macronucleus by genetic means is feasible. To investigate the relationship between the micronuclear map and coassortment groups, we systematically placed into CAGs all of the markers lying on chromosome 1L that are also found in the macronucleus. Sixteen CAGs were identified, 7 of which contain at least two loci. We have concluded that CAGs represent a fundamental genetic feature of the MAC. The MIC and MAC maps on 1L are colinear; that is, CAGs consist exclusively of markers that map to a continuous segment in a given region of the micronuclear map, with no intervening markers from other CAGs. These findings provide a solid foundation for exploiting the MAC chromosome pieces to build a physical map of the Tetrahymena genome.

High frequency intragenic recombination during macronuclear development in Tetrahymena thermophila restores the wild-type SerH1 gene

Macronuclear development in ciliates is characterized by extensive rearrangement of genetic material, including sequence elimination, chromosome fragmentation and telomere addition. Intragenic recombination is a relatively rare, but evolutionarily important phenomenon occurring in mitosis and meiosis in a wide variety of organisms. Here, we show that high frequency intragenic recombination, on the order of 30%, occurs in the developing amitotic macronucleus of the ciliate Tetrahymena thermophila. Such recombination, occurring between two nonsense transition mutations separated by 726 nucleotides, reproducibly restores wild-type expression of the SerH1 surface protein gene, thus mimicking complementation in trans heterozygotes. Recombination must be considered a potentially important aspect of macronuclear development, producing gene combinations not present in the germinal micronucleus.

Constitutive expression, not a particular primary sequence, is the important feature of the H3 replacement variant hv2 in Tetrahymena thermophila

Although quantitatively minor replication-independent (replacement) histone variants have been found in a wide variety of organisms, their functions remain unknown. Like the H3.3 replacement variants of vertebrates, hv2, an H3 variant in the ciliated protozoan Tetrahymena thermophila, is synthesized and deposited in nuclei of nongrowing cells. Although hv2 is clearly an H3.3-like replacement variant by its expression, sequence analysis indicates that it evolved independently of the H3.3 variants of multicellular eukaryotes. This suggested that it is the constitutive synthesis, not the particular protein sequence, of these variants that is important in the function of H3 replacement variants. Here, we demonstrate that the gene (HHT3) encoding hv2 or either gene (HHT1 or HHT2) encoding the abundant major H3 can be completely knocked out in Tetrahymena. Surprisingly, when cells lacking hv2 are starved, a major histone H3 mRNA transcribed by the HHT2 gene, which is synthesized little, if at all, in wild-type nongrowing cells, is easily detectable. Both HHT2 and HHT3 knockout strains show no obvious defect during vegetative growth. In addition, a mutant with the double knockout of HHT1 and HHT3 is viable while the HHT2 HHT3 double-knockout mutant is not. These results argue strongly that cells require a constitutively expressed H3 gene but that the particular sequence being expressed is not critical.

Germ-line knockout heterokaryons of an essential alpha-tubulin gene enable high-frequency gene replacement and a test of gene transfer from somatic to germ-line nuclei in Tetrahymena thermophila

The haploid Tetrahymena thermophila genome contains a single alpha-tubulin (ATU) gene. Using biolistic transformation, we disrupted one of the two copies of the ATU gene in the diploid germ-line micronucleus. The heterozygous germ-line transformants were made homozygous in the micronucleus by mating to a star strain containing a defective micronucleus. This mating, known as round 1 genomic exclusion, resulted in two heterokaryon clones of different mating types which have both copies of the ATU gene knocked out in the micronucleus but only wild-type genes in the polycopy somatic macronucleus. When these heterokaryons were mated, the exconjugant progeny cells did not grow because the new somatic macronuclei do not have any alpha-tubulin genes. However, when these conjugants were transformed with a functional marked ATU gene, viable transformants were obtained that contained the transforming ATU gene at the homologous locus in the new macronucleus. The exconjugant progeny could be rescued at a high efficiency (900 transformants per microg of DNA) with a wild-type ATU gene. Unlike previous macronuclear transformation protocols, this strategy should allow introduction of highly disadvantageous (but viable) mutations into Tetrahymena, providing a powerful tool for molecular and functional studies of essential genes. These knockout heterokaryons were used to demonstrate that gene transfer from somatic macronuclei to germ-line micronuclei occurs rarely if at all.

Essential and nonessential histone H2A variants in Tetrahymena thermophila

Although variants have been identified for every class of histone, their functions remain unknown. We have been studying the histone H2A variant hv1 in the ciliated protozoan Tetrahymena thermophila. Sequence analysis indicates that hv1 belongs to the H2A.F/Z type of histone variants. On the basis of the high degree of evolutionary conservation of this class of histones, they are proposed to have one or more distinct and essential functions that cannot be performed by their major H2A counterparts. Considerable evidence supports the hypothesis that the hv1 protein in T. thermophila and hv1-like proteins in other eukaryotes are associated with active chromatin. In T. thermophila, simple mass transformation and gene replacement techniques have recently become available. In this report, we demonstrate that either the HTA1 gene or the HTA2 gene, encoding the major H2As, can be completely replaced by disrupted genes in the polyploid, transcriptionally active macronucleus, indicating that neither of the two genes is essential. However, only some of the HTA3 genes encoding hv1 can be replaced by disrupted genes, indicating that the H2A.F/Z type variants have an essential function that cannot be performed by the major H2A genes. Thus, an essential gene in T. thermophila can be defined by the fact that it can be partially, but not completely, eliminated from the polyploid macronucleus. To our knowledge, this study represents the first use of gene disruption technology to study core histone gene function in any organism other than yeast and the first demonstration of an essential gene in T. thermophila using these methods. When a rescuing plasmid carrying a wild-type HTA3 gene was introduced into the T. thermophila cells, the endogenous chromosomal HTA3 could be completely replaced, defining a gene replacement strategy that can be used to analyze the function of essential genes.

Electroporation-mediated replacement of a positively and negatively selectable beta-tubulin gene in Tetrahymena thermophila

Replacement of lysine-350 by methionine in the beta-tubulin gene of Chlamydomonas confers resistance to microtubule-depolymerizing drugs and increased sensitivity to the microtubule-stabilizing drug taxol. This mutation was created in cloned BTU1, one of two coexpressed beta-tublin genes of Tetrahymena thermophila. When introduced by electroporation, the mutated gene transformed Tetrahymena exclusively by gene replacement at the homologous locus. Taxol-sensitive transformants could be retransformed with a wild-type gene and selection for taxol resistance. Analyses of phenotypic assortment and of the mRNA in transformed cells suggest that complete replacement of the BTU1 gene in the polyploid macronucleus can be obtained. These studies demonstrate the utility of this marker for studying tublin gene function and show that electroporation allows facile gene replacement in Tetrahymena.