The replication advantage of a free linear rRNA gene is restored by somatic recombination in Tetrahymena thermophila

The autonomously replicating rRNA genes (rDNA) in the somatic nucleus of Tetrahymena thermophila are maintained at a copy number of approximately 10(4) per nucleus. A mutant in which the replication properties of this molecule were altered was isolated and characterized. This mutation of inbred strain C3, named rmm4, was shown to have the same effect on rDNA replication and to be associated with the same 1-base-pair (bp) deletion as the previously reported, independently derived rmm1 mutation (D. L. Larson, E. H. Blackburn, P. C. Yaeger, and E. Orias, Cell 47:229-240, 1986). The rDNA of inbred strain B, which is at a replicational disadvantage compared with wild-type C3 rDNA, has a 42-bp deletion. This deletion is separated by 25 bp from the 1-bp deletion of rmm4 or rmm1. Southern blot analysis and DNA sequencing revealed that during prolonged vegetative divisions of C3-rmm4/B-rmm heterozygotes, somatic recombination produced rDNAs lacking both the rmm4-associated deletion and the 42-bp deletion. In somatic nuclei in which this rare recombinational event had occurred, all 10(4) copies of nonrecombinant rDNA were eventually replaced by the recombinant rDNA. The results prove that each of the two deletions is the genetic determinant of the observed replication disadvantage. We propose that the analysis of somatically recombinant rDNAs can be used as a general method in locating other mutations which affect rDNA propagation in T. thermophilia.

Replacement of the macronuclear ribosomal RNA genes of a mutant Tetrahymena using electroporation

The macronucleus of the ciliate Tetrahymena contains approx. 10(4) ribosomal RNA gene molecules (rDNA) in the form of linear, autonomously replicating palindromes. Previous studies have shown that macronuclear rDNA molecules derived from wild-type (wt) inbred strain C3 out-replicate those derived from wt inbred strain B, in macronuclei initially heterozygous for both, leading to the complete loss of the B rDNA. However, rmm-1, a cis-acting laboratory-induced mutation obtained previously by mutagenesis of inbred strain C3, causes the mutant rmm-1 rDNA to be completely out-replicated by B rDNA. These findings suggest the following hierarchy of replication potential: wt C3 greater than wt B greater than C3-rmm-1. We used electroporation to test whether cells containing only rmm-1 macronuclear rDNA are favorable recipients for transformation with either wt B or C3 donor rDNA molecules. The donor rDNA molecules carried the selectable marker Pmr (paromomycin resistance) located in the coding region of the 17S rRNA. Transformants were obtained, at a frequency greater than 1 in 10(5), by electroporation under a wide range of electrical discharge parameters. The fraction of cells surviving electroporation varied between 2 and greater than 95% in successful experiments. Replacement ('transplacement') of the recipient rDNA was observed, consistent with the prediction that B and C3 rDNA should out-replicate rmm-1 rDNA. These findings are also consistent with the previous conclusion that the differential replication determinants reside in the 5'-nontranscribed spacer of the rDNA.

Comparison of Tetrahymena ARS sequence function in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe

We have isolated several Tetrahymena thermophila chromosomal DNA fragments which function as autonomously replicating sequences (ARS) in the heterologous Saccharomyces cerevisiae and Schizosaccharomyces pombe selection systems. The Tetrahymena ARS sequences were first isolated in S. cerevisiae and were derived from non-ribosomal micro- and macronuclear DNA. Sequence analysis of the ARS elements identified either perfect or close matches with the 11 bp S. cerevisiae ARS core consensus sequence. Subcloning studies of two Tetrahymena ARS elements defined functional regions ranging in size from 50 to 300 bp. Testing of the ARS elements in S. pombe revealed that most of the T. thermophila inserts confer ARS function in both yeasts, at least in the sense of promoting a high transformation frequency to plasmids which contain them. However, the actual sequences responsible for ARS activity were not always identical in the two yeasts.

Molecular evidence for somatic recombination in the ribosomal DNA of Tetrahymena thermophila

The ribosomal DNA (rDNA) in Tetrahymena thermophila is a 21-kilobase-pair palindromic DNA molecule that replicates autonomously in the macronucleus and is maintained at the level of about 10,000 copies per macronucleus. The rDNA of inbred strain C3 outreplicates the rDNA of inbred strain B in most B/C3 heterozygous macronuclei, generating macronuclei containing exclusively C3 rDNA sequences. In 1% or less of the B/C3 heterozygous macronuclei, however, rDNA sequences derived from both B and C3 strains persist in the macronucleus (co-maintainers). We report here that long-term culture of co-maintainers has yielded recombinant rDNA molecules combining sequences from both parental inbred strains. The genetic structure of such molecules also gives us virtual certainty that the differential replication of C3 rDNA with respect to B rDNA is due to the DNA sequence difference previously reported in domain 2 of the rDNA replication regions of the two strains.

Pronuclear fusion failure: an alternate conjugational pathway in Tetrahymena thermophila, induced by vinblastine

Vinblastine is shown to induce pronuclear fusion failure in conjugating Tetrahymena thermophila. In this alternate conjugational pathway gametic pronuclei are exchanged between conjugants but do not fuse. Each pronucleus undergoes one mitotic division to produce a new macro- and micronucleus. Genetic consequences of pronuclear fusion failure include the following: (1) the progeny are whole genome homozygotes with nuclei derived from single meiotic products, and (2) half of the progeny are heterokaryons with micro- and macronuclei of different genetic origins. These facts make this process extremely useful in strain construction and mutant isolation. The induction of pronuclear fusion failure by vinblastine suggests that microtubules play an essential role in pronuclear fusion.

Genetic instability in the mating type system of Tetrahymena pigmentosa

Selfing clones of Tetrahymena pigmentosa show several interesting genetic features, and provide some insight into the mechanisms of mating type (mt) determination. They differ significantly from those of Tetrahymena thermophila. They are distributed nonrandomly in crosses. Their rates of stabilization are highly variable, but most are much lower than those reported for T. thermophila. A number of subclones derived from nearly all the selfers have maintained stable mts in culture for several years. However, some subclones manifest persistent selfing, long after the calculated completion of allelic assortment for heterozygous loci. This phenomenon along with the perpetual maintenance of dominant mts in heterozygotes shows that phenotypic assortment is not involved in mt expression. In crosses, many selfers exhibit quantitative and qualitative aberrations in the transmission of alleles to the gametes; some of the micronuclear changes underlying these aberrations occur during vegetative growth. There are rare illegitimate appearances of dominant alleles in sexual progeny, and more common illegitimate appearances of the most recessive phenotype.--Various models to explain mt determination in this species are considered. One which might account for the troubling phenomena of the system consists of an active mat expression site, with "cassettes" at other sites specific for the different dominant alleles and capable of transposition to the expression site.

Unidirectional co-stimulation by a non-mating strain of Tetrahymena thermophila

We report that a fatty acid auxotroph of Tetrahymena thermophila (RH179E1) fails to mate, yet retains the ability to co-stimulate normal cells unidirectionally. Thus, co-stimulation can be analyzed experimentally in the absence of pair formation. We show that the co-stimulation of normal cells of one mating type is sufficient to shorten the waiting period for pair formation of those cells with initiated cells. This is the first evidence that co-stimulation causes a hyperinduction of mating reactivity in T. thermophila, generating in turn a positive feedback mechanism for (presumably) gamone production. Co-stimulation by the variant strain is at a maximum after 3-4 h of exposure when the variant and wild-type cells are at a ratio of 1:1. When mixed with wild-type cells, RH179E1 induces the formation of progeny (at low frequency) which inherit exclusively genetic material of the wild-type cells.

A restriction fragment length polymorphism in the 5' non-transcribed spacer of the rDNA of Tetrahymena thermophila inbred strains B and C3

The ribosomal DNA of Tetrahymena thermophila is a 21-kb palindromic molecule which replicates autonomously in the macronuclei of this species. In addition to the rRNA coding regions, there are 5' and 3' flanking sequences which are not transcribed (non-transcribed spacer; NTS). The 5' NTS contains a bidirectional origin of DNA replication and promoter elements which direct transcription. We have identified a restriction fragment length polymorphism in the rDNA 5' NTS by comparing the B and C3 inbred strains of T. thermophila. There is a 42-bp region present in the C3 but not in the B strain rDNA; we present evidence that this difference most likely represents a deletion in the B strain rather than an insertion in the C3 strain. We also include a revised version of the nucleotide sequence of the 5' NTS DNA of inbred strain B.

Conjugation rescue of an exocytosis-competent membrane microdomain in Tetrahymena thermophila mutants

Conjugation-rescue experiments with two Tetrahymena thermophila mutants (exo-) incapable of exocytosis (SB255, SB258) have been used to dissect regulatory steps in assembly of a functional membrane microdomain, the fusion rosette. "Rescue" refers to the recovery of a secretory activity. Exo- mutants fail to secrete mucus normally (form capsules) when stimulated by the secretagogue alcian blue and are blocked before the assembly of a functional fusion rosette in the cell membrane. Two criteria are used to assay recovery of the wild-type (exo+) phenotype: the conjugant's ability to form capsules when stimulated and the presence of assembled rosettes, which disperse upon stimulation. Conjugation of exo+ X SB258 results in restoration of secretion in 60% of the mutant conjugants and reappearance of assembled rosettes. Secretory capacity is restored in the SB258 cell within one-half hour of firm pair formation. This restoration is not due to new gene expression or continued protein synthesis, since it occurs when SB258 is crossed to a "star" strain (A*), which has defective micronuclei and therefore cannot contribute wild-type genes, and restoration occurs in the presence of cycloheximide during conjugation. Conjugation of exo+ X SB255 reveals a real but inefficient restoration of exocytic capacity in the exo- conjugant and a significant decrease of exocytic capacity in the exo+ conjugant. SB255 X SB258 crosses also show a low but significant rescue of exocytic competence, indicating that different components of the exocytic mechanism are affected in the two mutants. This cross leads to restoration of rosette assembly and function in one of the partners, presumably SB258. These results provide data about some of the steps necessary for rosette assembly and suggest that transferable factors that promote and/or inhibit exocytosis are present in these cells.

Control of rDNA replication in Tetrahymena involves a cis-acting upstream repeat of a promoter element

A novel genetic scheme was used to isolate mutants altered in the formation or maintenance of amplified rDNA in the Tetrahymena macronucleus. One such mutant had a cis-acting rDNA mutation that affected the ability of mutant rDNA molecules to replicate in macronuclei in the presence of a wild-type (B strain) rDNA. The mutant rDNA was lost from these heterozygous macronuclei during vegetative cell divisions, although it was maintained normally in the homozygous or hemizygous state. In contrast, wild-type macronuclear rDNA of the C3 strain used to obtain the mutant outreplicated B strain rDNA in B/C3 heterozygote macronuclei. Sequence differences were found between wild-type B and C3 and mutant C3 rDNAs in the replication origin region, changing an upstream repeat of a highly conserved rRNA promoter element. We propose that the various rDNA alleles differentially compete for limiting amounts of trans-acting factors that bind to these enhancer-like repeats and positively regulate rDNA replication.

Developmental regulation of gene expression in Tetrahymena

The onset of gene expression in Tetrahymena thermophila during macronuclear differentiation was investigated by assay of galactokinase in conjugating deoxygalactose-resistant heterokaryons. Our results distinguish three successive states of galactokinase gene expression for cells developing a new macronucleus: stage 0, refractory to induction; stage 1, inducible by refeeding; and stage 2, induced. The refractory period ends at 12 to 13 hr after the onset of conjugation; this corresponds to the time of pair separation, and occurs several hours after the new macronuclei have become morphologically distinguishable. Stage 1 cells behave indistinguishably from mature starved cells. Inhibitor studies suggest that galactokinase synthesis is induced coincidentally with the induction of bulk protein synthesis during conjugation: thus it behaves developmentally like a typical protein; and that galactokinase mRNA is probably transcribed within 1 hr prior to its translation. Thus, when conjugating cells are refed during the refractory period, some developmental condition prevents the swift induction of protein (and galactokinase) synthesis observed upon refeeding starved (nonmating) cells. The possible nature of this developmental phenomenon is discussed.

Developmental and genetic effects of alcian blue in conjugating Tetrahymena thermophila: doublet formation and macronuclear retention

Genetic, kinetical and cortical effects of treatment with the inducer of mucocyst release, alcian blue (AB), on conjugating pairs of Tetrahymena thermophila are reported. AB induces the formation of doublet cells from pairs, and the majority of them are homopolar doublets. We present a model in order to explain the origin of these cells. Macronuclear retention (MR) is the most important genetic effect observed. Two kinds of MR can be obtained: prezygotic-MR (uniparental micronucleus) and postzygotic-MR (cross-fertilized micronucleus). Within the first group, both homokaryon and heterokaryon cells are obtained. From some abnormal conjugational configurations and the results of conjugational kinetic analysis we propose an explanation for the origin of MR cells induced by AB. Genetic effects obtained after AB treatment at different conjugational times are independent of the cortical ones. The utility of these different effects in genetical and physiological studies is discussed.

Isolation and ultrastructural characterization of secretory mutants of Tetrahymena thermophila

Isolation of 14-secretory mutants (exo-) of Tetrahymena thermophila and ultrastructural characterization (freeze-fracture and thin-section) of two of these (SB255 and SB258) are described. The site of secretion is marked by an intramembrane particle array, the rosette, beneath which the secretory organelle rests. Using Alcian Blue (8GS) as a secretagogue, a screening procedure for exo- cells was developed. Of the resulting 14 clones isolated, 10 are stable and have a tight mutant phenotype. Two of these, SB255 and SB258, lack assembled rosettes. Electron microscopy shows that SB255 has a reduced total number of mucocysts, whereas SB258 appears to have the normal number. This study demonstrates a useful eukaryotic model with which to study by genetic dissection the regulatory mechanisms involved in membrane events in secretion.

A microtubule meshwork associated with gametic pronucleus transfer across a cell-cell junction

In conjugating Tetrahymena, a cellular assembly composed of a microtubule meshwork appears to be required for the transfer of gametic pronuclei across the junction that separates the conjugating cells. This assembly is suggestive of a gametogenic cell division in ancient predecessors of ciliates, with Tetrahymena retaining only the associated nuclear division and export.

Allele-specific, selective amplification of a ribosomal RNA gene in Tetrahymena thermophila

The amplification of ribosomal DNA during development of the somatic macronucleus in Tetrahymena thermophila was analyzed by genetic and molecular biological techniques. We have identified an alternate form of the rDNA, structurally distinguishable from the wild-type by an extra cutting site for Bam HI in its nontranscribed spacer. The altered rDNA was inherited in crosses in a simple Mendelian fashion, consistent with the presence of only one rRNA gene copy per haploid genome in the micronucleus. We therefore define a locus for the rRNA structural gene, the rdnA locus, with the allele determining the alternate form designated rdnA1. In over 95% of T. thermophila clones heterozygous for the rdnA locus in the micronucleus (rdnA1/rdn+), the macronucleus, which develops from a division product of this micronucleus, contained almost exclusively rdnA1-type amplified palindromic rDNA molecules. The rdnA1 allele is thus almost always dominant over the rdn+ allele with respect to amplification. This genetic variant of the rdnA locus was used to show that the single, free, nonpalindromic rRNA genes, which are synthesized during rDNA amplification, are derived from micronuclear gene copies from both chromosomal homologs. We therefore conclude that in these heterozygotes, selective amplification of the rdnA1 allele is not caused by the production of only one type of free, single rRNA gene during amplification.

Phenylketonuric Tetrahymena: phenylalanine hydroxylase mutants and other tyrosine auxotrophs

Nineteen tyrosine auxotrophs of the ciliated protozoan Tetrahymena thermophila have been isolated and biochemically examined. These mutants are defective in the conversion of phenylalanine to tyrosine; this is analogous to the defect that causes phenylketonuria in humans. After nitrosoguanidine mutagenesis and self-fertilization, progeny clones were screened for tyrosine auxotrophy and positively identified by using growth tests and in vivo radiometric assays for phenylalanine-to-tyrosine conversion. Mutants in one complementation group (locus) lacked phenylalanine hydroxylase activity; mutants in three other loci appeared to be deficient in the unconjugated pteridine cofactor that is necessary for the function of the hydroxylase. Another mutant lacked the dihydropteridine reductase activity required to regenerate the reduced form of the pteridine cofactor. Because hydroxylation of tyrosine to dopa and of tryptophan to 5-hydroxytryptophan may require the same cofactor and pterin reductase as phenylalanine hydroxylase, these mutants may also prove useful for the study of the role of catecholamines and serotonin, substances known to be present in Tetrahymena.

Further evidence for lack of gene expression in the Tetrahymena micronucleus

Certain galA mutations in the ciliated protozoan Tetrahymena thermophila confer an almost total loss of galactokinase activity in homozygotes. Heterokaryons have been constructed that are homogeneous for the galA1 mutation in the (45n) macronucleus, but which contain a galA+ (2n) micronucleus. Soluble cell extracts prepared from these heterokaryons have been assayed for galactokinase activity, using a radiometric assay for the conversion of galactose to galactose-1-phosphate (gal-1-P). No galactokinase activity attributable to the micronuclear genes is observed in such heterokaryons. These results, obtained with the galA1 marker, provide the first direct, quantitative evidence for the lack of micronuclear (germ line) gene expression in Tetrahymena during vegetative growth, and substantiate the predictions of previous phenotypic observations on heterokaryons and autoradiographic studies of micronuclear RNA synthesis. The generality of this conclusion will be established in the future when other enzymically assayable mutations become available for similar studies.

CYTOGAMY: AN INDUCIBLE, ALTERNATE PATHWAY OF CONJUGATION IN TETRAHYMENA THERMOPHILA

We report the occurrence of cytogainy in Tetrahymena thermophila. By analogy to Paramecium, cytogamy generates exconjugant clones that derive their entire genetic information from a single meiotic product of their cytoplasmic parent. Thus, "instant" whole-genome homozygotes are created. Cytogamy has been induced in every strain of T.thermophila tested, and most of the excytogamous progeny have exhibited high fertility. The high frequency with which cytogamy can be induced by hyperosmotic shock, coupled with the foregoing genetic properties, make this process a practical (and already proven) method for the isolation of recessive mutants in T.thermophila. We also report that the cytogamy-inducing treatment induces other rare abnormalities of genetic transmission, which have not yet been characterized.

Dual capacity for nutrient uptake in Tetrahymena. V. Utilization of amino acids and proteins

We investigated the relative contributions of phagocytosis and plasma membrane transport to the uptake of amino acids and a protein (egg albumin) in amounts which allow Tetrahymena thermophila to grow and multiply. We used a mutant capable of indefinite growth without food vacuole formation (phagocytosis) and its wild type (phagocytosis-competent) isogenic parental strain. Our results suggest that phagocytosis is not required for free amino acid uptake, most or all of which can be attributed to carrier-mediated transport systems, apparently located on the plasma membrane. In contrast, phagocytosis is required for utilization of the protein. Proteins can supply required amino acids in amounts sufficient for growth only when food vacuoles are formed. We conclude that Tetrahymena thermophila either possesses no endocytic mechanisms at the cell surface other than food vacuole formation or, if it does, these putative mechanisms are not capable of nutritionally meaningful rates of protein uptake.

Osmotic shock prevents nuclear exchange and produces whole-genome homozygotes in conjugating Tetrahymena

Exposure of conjugating Tetrahymena to a hyperosmotic shock blocks the exchange of gametic nuclei and produces self-fertilized exconjugants that are homozygous for their whole genome. Cells are sensitive to this induction during a brief period after meiosis. The high efficiency of the treatment and the fertility of the progeny make this a useful method for the isolation of induced recessive mutations and enhances the value of Tetrahymena as an animal-cell model system in which genetic dissection is practical. The sharp peak of sensitivity is useful in the study of those cellular mechanisms responsible for the independent handling of several functionally distinct nuclei during conjugation.

Dual capacity for nutrient uptake in Tetrahymena. II. Role of the two systems in vitamin uptake

Previously, we found that a mutant strain of Tetrahymena pyriformis without food vacuoles failed to grow unless the nutrient media were richly supplemented with vitamins and trace metals. Here we show that calcium folinate alone can replace the extra vitamin supplementation. The mutant requires approximately 90-fold higher concentration of folinate than the wild-type cells to give similar growth responses in a chemically defined medium. We infer that the food vacuole is an important route of uptake for this vitamin in the wild-type cells. We found no difference between mutant and wild-type cells in their requirements for nicotinic acid, pantothenic acid, riboflavin-monophosphate, and pyridoxal. We infer that an extravacuolar route contributes importantly to uptake of these 4 compounds.

Tetrahymena: growth without phagocytosis

We have succeeded in growing a Tetrahymena mutant without food vacuoles in growth media supplemented with vitamins and heavy-metal salts. This finding implies the existence of adequate alternative routes of entry for every required nutrient, and clearly indicates that the food vacuole in Tetrahymena is a dispensable cellular organelle. The growth of the mutant without food vacuoles makes available a valuable experimental tool.