Permanent rescue of a non-Mendelian mutation of Paramecium by microinjection of specific DNA sequences

Programmed chromosome fragmentation in ciliated protozoa: multiple means to chromosome ends

SUMMARYCiliated protozoa undergo large-scale developmental rearrangement of their somatic genomes when forming a new transcriptionally active macronucleus during conjugation. This process includes the fragmentation of chromosomes derived from the germline, coupled with the efficient healing of the broken ends by de novo telomere addition. Here, we review what is known of developmental chromosome fragmentation in ciliates that have been well-studied at the molecular level (Tetrahymena, Paramecium, Euplotes, Stylonychia, and Oxytricha). These organisms differ substantially in the fidelity and precision of their fragmentation systems, as well as in the presence or absence of well-defined sequence elements that direct excision, suggesting that chromosome fragmentation systems have evolved multiple times and/or have been significantly altered during ciliate evolution. We propose a two-stage model for the evolution of the current ciliate systems, with both stages involving repetitive or transposable elements in the genome. The ancestral form of chromosome fragmentation is proposed to have been derived from the ciliate small RNA/chromatin modification process that removes transposons and other repetitive elements from the macronuclear genome during development. The evolution of this ancestral system is suggested to have potentiated its replacement in some ciliate lineages by subsequent fragmentation systems derived from mobile genetic elements.

Micromanipulation in Paramecium: From Non-Mendelian Inheritance to the Outlook for Versatile Micromachines

This review addresses nine areas of knowledge revealed by micromanipulations performed with Paramecium. Microinjection has shown that sexual maturation and senescence of Paramecium caudatum is a programmed process conducted by a specific gene and its product protein. In Paramecium tetraurelia, autogamy was revealed to depend on the number of DNA syntheses rather than the number of cell divisions in clonal aging. The cytoplasmic complementarity test established that microinjection of wild‐type cytoplasm can correct genetic defects of mutants. The concept of complementarity together with protein chemistry revealed compounds that control membrane excitability. In non‐Mendelian inheritance, noncoding small RNAs made from the parental micronucleus regulate the rearrangement of the progeny's macronuclear DNA. The macronucleus has the potential to be used as a factory for genetic engineering. The development and differentiation of progeny's nuclei in mating pairs are controlled by the parental macronucleus. The chemical reaction processes associated with exocytosis have been revealed by microinjection of various enzymes and antibodies. Using the fusion gene of histone H2B and yellow‐fluorescence protein, it was revealed that the fusion gene‐mRNA is transferred between cells during mating. Experiments with endosymbiotic bacteria and the host shed light on the conditions needed to establish sustainable symbiotic relationships.

Programmed Rearrangement in Ciliates: Paramecium

Programmed genome rearrangements in the ciliate Paramecium provide a nice illustration of the impact of transposons on genome evolution and plasticity. During the sexual cycle, development of the somatic macronucleus involves elimination of ∼30% of the germline genome, including repeated DNA (e.g., transposons) and ∼45,000 single-copy internal eliminated sequences (IES). IES excision is a precise cut-and-close process, in which double-stranded DNA cleavage at IES ends depends on PiggyMac, a domesticated piggyBac transposase. Genome-wide analysis has revealed that at least a fraction of IESs originate from Tc/mariner transposons unrelated to piggyBac. Moreover, genomic sequences with no transposon origin, such as gene promoters, can be excised reproducibly as IESs, indicating that genome rearrangements contribute to the control of gene expression. How the system has evolved to allow elimination of DNA sequences with no recognizable conserved motif has been the subject of extensive research during the past two decades. Increasing evidence has accumulated for the participation of noncoding RNAs in epigenetic control of elimination for a subset of IESs, and in trans-generational inheritance of alternative rearrangement patterns. This chapter summarizes our current knowledge of the structure of the germline and somatic genomes for the model species Paramecium tetraurelia, and describes the DNA cleavage and repair factors that constitute the IES excision machinery. We present an overview of the role of specialized RNA interference machineries and their associated noncoding RNAs in the control of DNA elimination. Finally, we discuss how RNA-dependent modification and/or remodeling of chromatin may guide PiggyMac to its cognate cleavage sites.

Programmed Genome Rearrangements in the Ciliate Oxytricha

The ciliate Oxytricha is a microbial eukaryote with two genomes, one of which experiences extensive genome remodeling during development. Each round of conjugation initiates a cascade of events that construct a transcriptionally active somatic genome from a scrambled germline genome, with considerable help from both long and small noncoding RNAs. This process of genome remodeling entails massive DNA deletion and reshuffling of remaining DNA segments to form functional genes from their interrupted and scrambled germline precursors. The use of Oxytricha as a model system provides an opportunity to study an exaggerated form of programmed genome rearrangement. Furthermore, studying the mechanisms that maintain nuclear dimorphism and mediate genome rearrangement has demonstrated a surprising plasticity and diversity of noncoding RNA pathways, with new roles that go beyond conventional gene silencing. Another aspect of ciliate genetics is their unorthodox patterns of RNA-mediated, epigenetic inheritance that rival Mendelian inheritance. This review takes the reader through the key experiments in a model eukaryote that led to fundamental discoveries in RNA biology and pushes the biological limits of DNA processing.

Genome-wide analysis of genetic and epigenetic control of programmed DNA deletion

During the development of the somatic genome from the Paramecium germline genome the bulk of the copies of ∼45 000 unique, internal eliminated sequences (IESs) are deleted. IES targeting is facilitated by two small RNA (sRNA) classes: scnRNAs, which relay epigenetic information from the parental nucleus to the developing nucleus, and iesRNAs, which are produced and used in the developing nucleus. Why only certain IESs require sRNAs for their removal has been enigmatic. By analyzing the silencing effects of three genes: PGM (responsible for DNA excision), DCL2/3 (scnRNA production) and DCL5 (iesRNA production), we identify key properties required for IES elimination. Based on these results, we propose that, depending on the exact combination of their lengths and end bases, some IESs are less efficiently recognized or excised and have a greater requirement for targeting by scnRNAs and iesRNAs. We suggest that the variation in IES retention following silencing of DCL2/3 is not primarily due to scnRNA density, which is comparatively uniform relative to IES retention, but rather the genetic properties of IESs. Taken together, our analyses demonstrate that in Paramecium the underlying genetic properties of developmentally deleted DNA sequences are essential in determining the sensitivity of these sequences to epigenetic control.

DNA elimination in ciliates: transposon domestication and genome surveillance

Ciliated protozoa extensively remodel their somatic genomes during nuclear development, fragmenting their chromosomes and removing large numbers of internal eliminated sequences (IESs). The sequences eliminated are unique and repetitive DNAs, including transposons. Recent studies have identified transposase proteins that appear to have been domesticated and are used by these cells to eliminate DNA not wanted in the somatic macronucleus. This DNA elimination process is guided by meiotically produced small RNAs, generated in the germline nucleus, that recognize homologous sequences leading to their removal. These scan RNAs are found in complexes with PIWI proteins. Before they search the developing genome for IESs to eliminate, they scan the parental somatic nucleus and are removed from the pool if they match homologous sequences in that previously reorganized genome. In Tetrahymena, the scan RNAs target heterochromatin modifications to mark IESs for elimination. This DNA elimination pathway in ciliates shares extensive similarity with piRNA-mediated silencing of metazoans and highlights the remarkable ability of homologous RNAs to shape developing genomes.

RNA-directed epigenetic regulation of DNA rearrangements

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.

Epigenetic inheritance in ciliates

2009 marks not only the 200th anniversary of Darwin's birth but also publication of the first scientific evolutionary theory, Lamarck's Philosophie Zoologique. While Lamarck embraced the notion of the inheritance of acquired characters, he did not invent it (Burkhardt, 1984). New phenomena discovered recently offer molecular pathways for the transmission of several acquired characters. Ciliates have long provided model systems to study phenomena that bypass traditional modes of inheritance. RNA, normally thought of as a conduit in gene expression, displays a novel mode of action in ciliated protozoa. For example, maternal RNA templates provide both an organizing guide for DNA rearrangements in Oxytricha and a template that can transmit spontaneous mutations that may arise during somatic growth to the next generation, providing two such mechanisms of so-called Lamarckian inheritance. This suggests that the somatic ciliate genome is really an 'epigenome', formed through templates and signals arising from the previous generation. This review will discuss these new biological roles for RNA, including non-coding 'template' RNA molecules. The evolutionary consequences of viable mechanisms in ciliates to transmit acquired characters may create an additional store of heritable variation that contributes to the cosmopolitan success of this diverse lineage of microbial eukaryotes.

Developmental genome rearrangements in ciliates: a natural genomic subtraction mediated by non-coding transcripts

Several classes of non-protein-coding RNAs have recently been identified as epigenetic regulators of developmental genome rearrangements in ciliates, providing an interesting insight into the role of genome-wide transcription. In these unicellular eukaryotes, extensive rearrangements of the germline genome occur during the development of a new somatic macronucleus from the germline micronucleus. Rearrangement patterns are not dictated by the germline sequence, but reproduce the pre-existing rearrangements of the maternal somatic genome, implying a homology-dependent global comparison of germline and somatic genomes. We review recent evidence showing that this is achieved by a natural genomic subtraction, computed by pairing interactions between meiosis-specific, germline scnRNAs (small RNAs that resemble metazoan piRNAs) and longer non-coding transcripts from the somatic genome. We focus on current models for the RNA-based mechanisms enabling the cell to recognize the germline sequences to be eliminated from the somatic genome and to maintain an epigenetic memory of rearrangement patterns across sexual generations.

Posttranscriptional control is a strong factor enabling exclusive expression of surface antigens in Paramecium tetraurelia

Variable antigens are large proteins located on the outer membrane of parasitic but also of free-living protists. Multigene families encoding surface antigens demonstrate an exclusive expression of proteins. The resulting presence of just one protein species on the cell surface is required for surface antigen function; therefore, the molecular mechanism of exclusive expression is of main interest. Regulation of gene expression and mechanisms establishing switching of antigens are hardly understood in any organism. Here we report on the reaction of Paramecium to the artificial knock down of surface antigen 51A expression by bacteria-mediated RNAi. This technique involves the feeding of dsRNA-producing bacteria. We analyzed different fragments of the target gene for dsRNA template regarding their specific knock down efficiency and found great differences. Treatment of Paramecia with RNAi against the 51A antigen demonstrated that although a massive amount of mRNA was present, the protein was not detected on the cell surface. Moreover, a minor abundance of 51D transcripts resulted in an exclusive presence of 51D proteins on the cell surface. This posttranscriptional regulation was confirmed by the transcript ratio (51A/51D) determined by real-time (RT) PCR of single cells. Because we were able to document unexclusive transcription also in wild-type cells our results indicate that this posttranscriptional regulation is a main factor of enabling exclusive gene expression. The comparison of serotype shifts, caused by efficient and inefficient knock down, indicates an involvement of full-length transcripts in regulation of gene expression. Thus, our study gives new insights into the mechanism of exclusive expression on the molecular level: (i) exclusive transcription does not occur, (ii) posttranscriptional regulation is a powerful factor enabling exclusive antigen expression, and (iii) surface antigen mRNA is shown to be involved in this mechanism in a regulating way.

Épigénétique : la paramecia comme modèle d’étude

Since the middle of the last century, Paramecium has appeared as an intriguing genetic model, displaying a variety of heritable characters which do not follow the Mendel laws but are cytoplasmically inherited. The analysis of the hereditary mechanisms at play in this eukaryotic unicellular organism has provided new insight into epigenetics mechanisms. Interestingly, the revealing phenomena concern two pecularities of Paramecium, its highly elaborate surface structure (with thousands of ciliary basal bodies as cytoskeleton organizers), and its nuclear dualism (coexistence of a diploid "germline" micronucleus and a highly polyploid somatic macronucleus devoted to transcription, which contains a rearranged version of the germline genome). Analysis of variant cortical organization has led to the concept of structural inheritance, implying that assembly of new organelles and supramolecular protein complexes is guided by pre-existing organization. Analysis of other cytoplasmically inherited characters revealed that the developing macronucleus is epigenetically programmed by the maternal macronucleus through RNA-mediated, homology-dependent effects, suggesting the transcriptome should be recognized as a third actor in cellular inheritance, along with the "structurome" and the genome.

RNA-mediated programming of developmental genome rearrangements in Paramecium tetraurelia

The germ line genome of ciliates is extensively rearranged during development of the somatic macronucleus. Numerous sequences are eliminated, while others are amplified to a high ploidy level. In the Paramecium aurelia group of species, transformation of the maternal macronucleus with transgenes at high copy numbers can induce the deletion of homologous genes in sexual progeny, when a new macronucleus develops from the wild-type germ line. We show that this trans-nuclear effect correlates with homology-dependent silencing of maternal genes before autogamy and with the accumulation of approximately 22- to 23-nucleotide (nt) RNA molecules. The same effects are induced by feeding cells before meiosis with bacteria containing double-stranded RNA, suggesting that small interfering RNA-like molecules can target deletions. Furthermore, experimentally induced macronuclear deletions are spontaneously reproduced in subsequent sexual generations, and reintroduction of the missing gene into the variant macronucleus restores developmental amplification in sexual progeny. We discuss the possible roles of the approximately 22- to 23-nt RNAs in the targeting of deletions and the implications for the RNA-mediated genome-scanning process that is thought to determine developmentally regulated rearrangements in ciliates.

Non-Mendelian inheritance induced by gene amplification in the germ nucleus of Paramecium tetraurelia

A genetic investigation of strain d4-95, which carries a recessive mutant allele (pwB(95)) of pawn-B, one of the controlling elements of voltage-dependent calcium channels in Paramecium tetraurelia, revealed a non-Mendelian feature. Progeny of the cross between d4-95 and wild type often expressed a clonally stable mutant phenotype, even when they had a wild-type gene. The mutant phenotype was also expressed after self-fertilization of theoretical wild-type homozygotes recovered from the cross. Our molecular analysis demonstrated that the copy number of the mutant pwB gene in the micro- and macronucleus of d4-95 was much greater than that of the wild type. Most of the amplified, extra pwB gene copies in d4-95 were heritable independently from the original pwB locus. Repeated backcrossing of d4-95 with the wild type to dilute extra pwB genes in the strain produced segregants with a completely normal Mendelian trait in testcrosses. These results strongly suggest that a non-Mendelian inheritance of d4-95 was induced by gene amplification in the micronucleus.

Large-scale genome remodelling by the developmentally programmed elimination of germ line sequences in the ciliate Paramecium

In Paramecium, during the development of the somatic macronucleus, precise excision of thousands of single-copy non-coding sequences is initiated by specific DNA double-strand breaks, while imprecise elimination of germ-line-limited repeated sequences leads to internal deletions or chromosome fragmentation. Recent data point to a role of non-coding RNAs in the epigenetic programming of these rearrangements.

A member of a repeat family is the source of an insertion-deletion polymorphism inside a developmentally eliminated sequence of Tetrahymena thermophila

In Tetrahymena thermophila, the development of a transcriptionally active macronucleus from a transcriptionally inert micronucleus is accompanied by the elimination of numerous DNA segments, called internally eliminated sequences (IESs), many of which belong to dispersed repetitive sequence families. To examine the relationship between the insertion and deletion events expected to occur during evolution of the repeats and the developmental elimination process, IESs were compared among different Tetrahymena strains. A 600 base-pair DNA segment, the R Indel, was discovered inside the R IES, one of the ten sequenced IESs out of an estimated 6000 total in the Tetrahymena genome. The R Indel was found in strains B3 and C2 but not in several other strains examined, indicating that the Indel was probably present in a progenitor of strains B3 and C2. The R Indel was found to belong to a moderately large sequence family of about 200 members; however, BLAST searches did not reveal meaningful similarities with other mobile elements. Sequence comparisons revealed that a 300 base-pair stretch, very closely related to the first half of the R Indel, was present inside the previously described B IES, another of the ten sequenced IESs. This is the first example of shared sequences between two of the known IESs.

Genome remodeling in ciliated protozoa

The germline genomes of ciliated protozoa are dynamic structures, undergoing massive DNA rearrangement during the formation of a functional macronucleus. Macronuclear development involves chromosome fragmentation coupled with de novo telomere synthesis, numerous DNA splicing events that remove internal segments of DNA, and, in some ciliates, the reordering of scrambled gene segments. Despite the fact that all ciliates share similar forms of DNA rearrangement, there appears to be great diversity in both the nature of the rearranged DNA and the molecular mechanisms involved. Epigenetic effects on rearrangement have also been observed, and recent work suggests that chromatin differentiation plays a role in specifying DNA segments either for rearrangement or for elimination.

Epigenetic mechanisms affecting macronuclear development in Paramecium and Tetrahymena

Epigenetic inheritance includes all non-Mendelian inheritance, in fact any inheritance that does not arise from base changes. Ciliates, particularly Paramecium and Tetrahymena, undergo epigenetic changes to their macronuclei when they are formed at nuclear reorganization. Once set, however, they are reproduced in a constant fashion, except for allelic segregations, during vegetative fissions in Tetrahymena and certain life cycle changes in both Paramecium and Tetrahymena. This review is meant to be inclusive, discussing all the known cases of epigenetic changes in macronuclei. They involve virtually all traits. We find that these macronuclear changes are subject to a variety of modifications in the way that they are implemented. They constitute a major feature of ciliate genetics, probably because the separation of generative and vegetative functions to micronuclei and macronuclei makes such changes possible.

Sequence-specific epigenetic effects of the maternal somatic genome on developmental rearrangements of the zygotic genome in Paramecium primaurelia

In ciliates, the germ line genome is extensively rearranged during the development of the somatic macronucleus from a mitotic product of the zygotic nucleus. Germ line chromosomes are fragmented in specific regions, and a large number of internal sequence elements are eliminated. It was previously shown that transformation of the vegetative macronucleus of Paramecium primaurelia with a plasmid containing a subtelomeric surface antigen gene can affect the processing of the homologous germ line genomic region during development of a new macronucleus in sexual progeny of transformed clones. The gene and telomere-proximal flanking sequences are deleted from the new macronuclear genome, although the germ line genome remains wild type. Here we show that plasmids containing nonoverlapping segments of the same genomic region are able to induce similar terminal deletions; the locations of deletion end points depend on the particular sequence used. Transformation of the maternal macronucleus with a sequence internal to a macronuclear chromosome also causes the occurrence of internal deletions between short direct repeats composed of alternating thymines and adenines. The epigenetic influence of maternal macronuclear sequences on developmental rearrangements of the zygotic genome thus appears to be both sequence specific and general, suggesting that this trans-nucleus effect is mediated by pairing of homologous sequences.

Non-Mendelian inheritance of macronuclear mutations is gene specific in Paramecium tetraurelia

Paramecium tetraurelia contains two types of nuclei, a diploid germinal micronucleus and a large transcriptionally active macronucleus. The macronuclear genome is formed from the micronuclear DNA during sexual reproduction. Previous studies have shown that the processing of the A-type variable surface protein gene during formation of a new macronucleus is dependent on the presence of the A gene in the old macronucleus. It is not clear if this is a general feature that controls the formation of the Paramecium macronuclear genome or a unique feature of the A locus. Using micronuclear transplantation, we have constructed a strain that has a wild-type micronucleus but has macronuclear deletions of the A- and B-type surface protein genes. Neither the A nor the B gene is incorporated into the new macronucleus after sexual reproduction. Macronuclear transformation of this strain with the B gene rescues the B-gene deletion after formation of the next macronucleus but has not effect on the A deletion. Similarly, transformation with the A gene shows gene-specific rescue for A but not B. The effect of the old macronucleus on the processing of the new macronucleus results in a pattern of non-Mendelian inheritance of both macronuclear deletions. Progeny from the wild-type exconjugant are all wild type, and progeny from the A- B- exconjugant are mutant. The features of this A- B- non-Mendelian mutant demonstrate that the regulation of macronuclear DNA processing is gene specific, and our results open the possibility that this type of regulation affects many regions of the Paramecium genome.

The role of macronuclear DNA sequences in the permanent rescue of a non-mendelian mutation in Paramecium tetraurelia

The paramecium tetraurelia mutant called d48 has a complete copy of the A surface protein gene in its micronuclei, but lacks the A gene in the macronucleus. Previous experiments have shown that microinjection of a plasmid containing the entire A gene or a large portion of the gene into the macronucleus of d48 rescued the cell line after formation of a new macronucleus (autogamy). Here we show that several different regions of the A gene can rescue d48, but 100% of the activity cannot be localized to a single, defined region. Inversion of a sequence contained within an A gene plasmid had no measurable effect on rescue efficiency and co-injection of two different plasmids results in enhancement of rescue activity despite the non-contiguous form of the DNA sequences. Both these results suggest that no specific product (RNA or protein) with defined end points is made from the rescuing fragment. A unique restriction site was created in the A gene and used to demonstrate that the injected DNA does not serve as a direct template for the synthesis of the new macronuclear DNA. Models to explain the action of the injected DNA are discussed.

Identification of DNA segments capable of rescuing a non-mendelian mutant in paramecium

The non-Mendelian mutant d48 of Paramecium tetraurelia contains micronuclear wild type A genes, but at autogamy and conjugation proper processing fails and new macronuclei lack A genes. When cloned A genes are injected into the macronucleus of d48, proper processing is restored at the next autogamy; d48 is rescued, becoming permanently wild type. In the present study we have injected portions of the A gene into d48. We find that the ability to rescue extends over a large portion of the gene, with highest activity near a series of 221-bp repeat units in the middle of the gene. Regions outside the A gene are inactive.

Non-Mendelian inheritance of macronuclear mutations is gene specific in Paramecium tetraurelia

Paramecium tetraurelia contains two types of nuclei, a diploid germinal micronucleus and a large transcriptionally active macronucleus. The macronuclear genome is formed from the micronuclear DNA during sexual reproduction. Previous studies have shown that the processing of the A-type variable surface protein gene during formation of a new macronucleus is dependent on the presence of the A gene in the old macronucleus. It is not clear if this is a general feature that controls the formation of the Paramecium macronuclear genome or a unique feature of the A locus. Using micronuclear transplantation, we have constructed a strain that has a wild-type micronucleus but has macronuclear deletions of the A- and B-type surface protein genes. Neither the A nor the B gene is incorporated into the new macronucleus after sexual reproduction. Macronuclear transformation of this strain with the B gene rescues the B-gene deletion after formation of the next macronucleus but has not effect on the A deletion. Similarly, transformation with the A gene shows gene-specific rescue for A but not B. The effect of the old macronucleus on the processing of the new macronucleus results in a pattern of non-Mendelian inheritance of both macronuclear deletions. Progeny from the wild-type exconjugant are all wild type, and progeny from the A- B- exconjugant are mutant. The features of this A- B- non-Mendelian mutant demonstrate that the regulation of macronuclear DNA processing is gene specific, and our results open the possibility that this type of regulation affects many regions of the Paramecium genome.

DNA sequence requirements for the regulation of immobilization antigen A expression in Paramecium tetraurelia

The Paramecium surface proteins (immobilization antigens) are expressed in a mutually exclusive manner; only one antigen is found on the cell surface at a time. Expression of these proteins is regulated in response to environmental cues such as temperature and pH. This regulation has been shown to be controlled at the level of mRNA abundance by transcriptional and post-transcriptional mechanisms. Here, we have studied the transcription and regulated expression of the immobilization antigen A gene in Paramecium tetraurelia by transforming an A-deficient strain, d12, with cloned portions of the A gene via microinjection. The A gene is approximately 8 kilobases (kb) long with the transcription start site at position -9 or -8 and the start of translation at position +1. Paramecia transformed with cloned DNA containing A-gene sequences beginning at position -264 and ending 63 base pairs (bp) past the gene's polyadenylation site show properly regulated expression of immobilization antigen A. Lines derived from paramecia transformed with a plasmid containing A-gene sequences starting at position -211, however, show markedly reduced A-gene mRNA levels, and rarely express the A antigen. Nevertheless, cells that do express the A protein exhibit mutual exclusion and normal responses to environmental stimuli. Thus, the 54 bp between -264 and -211, while important for transcription, are not involved in the control of mutual exclusion and responses to environmental changes. Further deletion to position -151 yields similar, but more extreme, results.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular and genetic analyses of the B type surface protein gene from Paramecium tetraurelia

The gene encoding the B type variable surface protein from Paramecium tetraurelia stock 51 has been cloned and sequenced. The 7,182 nucleotide open reading frame contains no introns and encodes a cysteine-rich protein that has a periodic structure including three nearly perfect tandem repeats in the central region. Interestingly, the B gene is located near a macronuclear telomere as was shown previously for two other paramecium surface protein genes. In this paper, we characterize four independent mutants with complete macronuclear deletions of the B gene. Previous analysis of different macronuclear deletion mutants of the A surface protein gene demonstrated two types of inheritance: typical Mendelian segregation (as illustrated by d12) and cytoplasmic inheritance (shown by d48). F1 analysis of four B- mutants crossed with wild-type cells reveals heterozygous F1 cell lines derived from both parental cytoplasms contain approximately the same copy number of the B gene, as expected for a recessive Mendelian mutation. Analysis of F2 progeny from three of these four B- mutant crosses indicates that one of the three exhibits a Mendelian 1:1 segregation ratio of B+ and B- cell lines. The other two show a preponderance of B+ cells, but this is not correlated with the parental cytoplasmic type. In addition to having a large number of B+ individuals, the d12.144, A-, B- mutant produced some F2 progeny that stably maintain less than normal macronuclear amounts of the A gene and/or the B gene.