mRNAs for the immobilization antigens of Paramecium

Immobilization antigens of stock 51 of Paramecium tetraurelia were subjected to electrophoresis in NaDodSO(4)/polyacrylamide gels. Type A is estimated to have a molecular size of 300,000 daltons; H is estimated to be 288,000, D to be 280,000, E to be 270,000, B to be 253,000, and C to be 250,000. Poly(A)(+)RNAs have been isolated from cells producing these antigens and subjected to electrophoresis in methylmercury gels. A major band is found to vary in mobility with antigenic type: Its position in preparations derived from paramecia synthesizing antigen A indicates a size of 8400 nucleotide residues; its position from paramecia synthesizing other antigens indicate H, 8200; D, 7900; E, 7500; B, 7600; and C, 7000. Because of the sizes and quantities of these RNAs, it is argued that they probably represent the mRNAs for the immobilization antigens. It is concluded that each immobilization antigen probably consists of a single polypeptide and that only one major serotype-determining mRNA is present in each antigenically different paramecium.

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.

Does ribosomal DNA get out of the micronuclear chromosome in Paramecium tetraurelia by means of a rolling circle?

The macronuclear genes coding for rRNA (ribosomal DNA [rDNA]) of Paramecium tetraurelia, stock 51, are arranged in polymers consisting of units made up of a transcribed coding region and a nontranscribed spacer region. The whole macronuclear polymer ends with a portion of the spacer on either end followed by a telomere. Six kinds of macronuclear units, or genes, were mapped. Spacers were different, and transcribed regions were the same. These genes are found in markedly different numbers in the macronucleus. The most common gene shows two regions in the spacer where a sequence is followed by a direct repeat. The next most common gene is similar but shows a deletion plus a number of base pair substitutions. Although most cosmid clones contain only a single kind of gene, many contain more than one. These are thought to be produced by somatic crossing over. The four micronuclear genes that have been isolated consist of a single central transcribed region and portions of the spacer on either end. Sequencing indicates that the two ends of the molecule are partially redundant. While the spacer region at the right end of the macronuclear polymer is derived from the micronuclear spacer on the right, the spacer at the left end of the macronuclear polymer is derived from regions of the micronuclear spacer on both the right and the left. To account for this situation, a rolling-circle model for generation of the macronuclear rDNA from the micronuclear DNA is proposed.

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.

Developmentally excised sequences in micronuclear DNA of Paramecium

DNA processing occurs in ciliates at autogamy and conjugation when new macronuclei are formed from micronuclei and old macronuclei degrade. Processing of micronuclear DNA consists of removal of certain internal sequences, chromosomal fragmentation, addition of new telomeres, and amplification. Aside from a recent brief report, internal eliminated sequences have not been described in Paramecium. In this paper we characterize nine internal eliminated sequences found within and near the gene that codes for surface protein A in Paramecium tetraurelia. Of these nine, seven are located within the translated portion of the gene, and all include short, inverted terminal repeats. The characteristic sequence, TA, appears at the boundaries of all of the internal eliminated sequences.

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)

Micronuclear DNA from Paramecium tetraurelia: serotype 51 A gene has internally eliminated sequences

A method for the isolation of micronuclear DNA from Paramecium tetraurelia has been developed. After cell lysis, a low speed centrifugation at 1,000 g is used to remove all of the unbroken cells and macronuclei and approximately two thirds of the macronuclear fragments. Next a higher speed centrifugation of 9,000 g sediments the micronuclei and frees them from small particulates and soluble constituents. Advantage is then taken of the fact that micronuclei have a lower density than do macronuclear fragments in 45%-60% Percoll. Micronuclei float to the top during centrifugation at 24,000 g, while macronuclear fragments sediment. After several cycles of centrifugation in Percoll, the micronuclei, although heavily contaminated with cytoplasmic components, are essentially free of macronuclei and macronuclear fragments. Micronuclear DNA can then be extracted from the suspension. The whole procedure is very rapid and in about an hour micronuclear and macronuclear DNA can be separated. About 2 micrograms of micronuclear DNA can be obtained from 6 x 10(7) paramecia. We find that there are internal sequences in the micronuclear A gene DNA in wild type cells which are eliminated when the micronuclei develop into macronuclei. They yield unique restriction fragments for micronuclei and macronuclei. Therefore the purity of the preparations is easily monitored by probing Southern blots of restriction enzyme-digested DNA with the cloned A gene. No differences have been found between the micronuclear A gene in wild type and the d48 mutant.

Ciliate developmental genetics

We show that bacteriophage lambda DNA fragments microinjected into the macronucleus of the ciliated protozoan Paramecium can replicate as unit-length linear molecules. These linear DNA molecules are substrates for the addition of Paramecium telomeres by an endogenous telomerase. The linear DNA pieces can exist at copy numbers much higher than that of typical endogenous macronuclear chromosomes. We show that the copy number of injected DNA many fissions after microinjection reflects that of the original input copy number, suggesting that active control of copy number does not occur. Instead, the results suggest that injected DNA is replicated once per cell division.

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

The mutant Paramecium tetraurelia cell line d48 is unable to express the serotype A protein on its surface. Although the A gene is intact in the micronuclei of d48, the A gene copies in the macronucleus contain a large deletion eliminating virtually the entire coding sequence. Previous studies showed that microinjection of a plasmid containing the entire A gene into the macronucleus of d48 permanently restored A expression after autogamy. Together with other data, this result suggests that in wild type cells the A gene in the old macronucleus ensures the presence of a cytoplasmic factor that prevents A gene deletions at autogamy. In d48, where there are few, if any copies of the intact A gene in the old macronucleus, deletions occur during macronuclear formation. To elucidate the specific molecular mechanisms involved in this unusual phenomenon, we attempted to define the region(s) of the A gene necessary for rescuing d48. We show that microinjection of a 4.5-kb internal A gene fragment is sufficient for proper processing at autogamy and leads to permanent rescue of d48; i.e., the rescued strain is indistinguishable from wild type. Thus, rescue of d48 does not require upstream transcriptional control sequences, intact A mRNA or A serotype protein. We also show that various fragments of the A gene have the ability to rescue d48 to different extents, some being more efficient than others. We find no evidence to suggest that the A gene gives rise to a small stable RNA that might act as or encode a cytoplasmic factor. Molecular mechanisms that may be involved in the rescue of d48 are discussed.

Mutants affecting processing of DNA in macronuclear development in paramecium

In Paramecium tetraurelia, stock 51, the A surface protein is coded by the wild type A51 gene, present in micronuclei in two copies and in macronuclei in about 1500 copies. DNA processing, comprised of DNA cleavage, copy number amplification and telomere addition occurs at autogamy and conjugation when old macronuclei degrade and new macronuclei are formed from micronuclei. In this paper we characterize mutants with macronuclear A gene deletions. These mutants are notable in three respects. First, the mutants do not appear to be simple micronuclear deletions. Although genetic analysis shows that the d12 mutant d12(-1300) is homozygous for the allele A-1300 and the mutant d12(+1) for A+1, analysis by the polymerase chain reaction indicates that the micronuclei in these two mutants contain intact, but presumably altered, micronuclear A genes. They undergo deletion during DNA processing when new macronuclei are formed. Second, the position of the deletions in these alleles has been shown to change. The deficiency present in the d12 allele A-1300 was originally determined to extend from position -1300 (relative to the start of translation of the A gene) to the end of the chromosome. Later, a derivative of this strain, homozygous for the d12 allele A+1 was isolated in which the start site of the deletion was found to have moved from -1300 to +1. Third, a surprising interaction occurs in crosses between a line homozygous for the d12 allele and one homozygous for the wild-type A51 allele. Previous work on the non-Mendelian d48 mutant (which has intact A51 genes in its micronucleus, but has truncated A51 genes in its macronucleus) has shown that intact A51 alleles must be present in the old macronucleus in order for A51 alleles to undergo proper processing. We find that d12 alleles act on A51 alleles in heterozygotes such that intact macronuclear A genes are no longer required for proper processing of A51. Thus, in crosses of 51 x d12 (either +1 or -1300) d12 exconjugants, as well as 51 exconjugants, give rise to clones carrying both intact A51 and truncated d12 alleles. Remarkably the d12 alleles, which are themselves deleted during processing, are capable in the heterozygote of fostering normal processing of the A51 allele.

Multilevel regulation of surface antigen gene expression in Paramecium tetraurelia

A family of genes is responsible for production of surface antigenic components of Paramecium tetraurelia. These surface proteins are expressed in a mutually exclusive manner. Individuals rarely display more than one type. However, changes in environmental conditions can cause different surface proteins which replace preexisting types to be expressed. We investigated the nature of regulation of the genes for the A, C, and H surface antigens of P. tetraurelia. A system for in vitro run-on transcription was developed from crude Paramecium extracts and used in this analysis. The genes for surface antigens A and H were controlled at the level of transcription. However, the gene for surface antigen C demonstrated both transcriptional and posttranscriptional control, depending on the serotype being expressed. When animals expressed serotype A, the gene for surface antigen C was not transcribed. However, when animals expressed serotype H, the gene for surface antigen C was actively transcribed and stable surface antigen C mRNA was present in the cells, although surface antigen C was not detectable by serotype testing or by a salt-alcohol extraction method. The kinetics of transformation from serotype H to serotype C were determined by using the in vitro transcription system and monitoring steady-state RNA levels. During the transition, serotype A transcription was detected in run-on transcription experiments, although this RNA did not accumulate. The results indicate that serotype expression is controlled at several levels and that not all serotype genes are controlled in the same manner.

Autonomous replication and addition of telomerelike sequences to DNA microinjected into Paramecium tetraurelia macronuclei

Paramecium tetraurelia can be transformed by microinjection of cloned serotype A gene sequences into the macronucleus. Transformants are detected by their ability to express serotype A surface antigen from the injected templates. After injection, the DNA is converted from a supercoiled form to a linear form by cleavage at nonrandom sites. The linear form appears to replicate autonomously as a unit-length molecule and is present in transformants at high copy number. The injected DNA is further processed by the addition of paramecium-type telomeric sequences to the termini of the linear DNA. To examine the fate of injected linear DNA molecules, plasmid pSA14SB DNA containing the A gene was cleaved into two linear pieces, a 14-kilobase (kb) piece containing the A gene and flanking sequences and a 2.2-kb piece consisting of the procaryotic vector. In transformants expressing the A gene, we observed that two linear DNA species were present which correspond to the two species injected. Both species had Paramecium telomerelike sequences added to their termini. For the 2.2-kb DNA, we show that the site of addition of the telomerelike sequences is directly at one terminus and within one nucleotide of the other terminus. These results indicate that injected procaryotic DNA is capable of autonomous replication in Paramecium macronuclei and that telomeric addition in the macronucleus does not require specific recognition sequences.

Molecular biology of the genes for immobilization antigens in Paramecium

Several genes for surface antigens of the Paramecium aurelia complex of species have been isolated. In addition to known deletions of the 51A gene, we have obtained deletions involving the 51B gene and have developed a procedure for obtaining deletions of additional genes. Both Mendelian and non-Mendelian deletions of both the A and B genes have been found. In the non-Mendelian deletions the genes are present in the micronuclei and absent in the macronuclei. Processing of micronuclear DNA into new macronuclear DNA at conjugation and autogamy is under the control of the old macronucleus, and newly forming macronuclei become exactly like the old. Thus in the non-Mendelian mutants, macronuclei have a specific antigen gene deleted and also are impaired in their ability to direct normal DNA processing at the next conjugation or autogamy. These cases, along with others, show that this system of macronuclear control is a fundamental feature of ciliate genetics. The sequence of the 51A and 51C genes is described and compared with the 156G and 51H genes obtained by others. The 51A and 156G genes are remarkably similar while 51C and 51H are rather different. No introns or pseudogenes have been observed. Some, possibly all, of the genes are on the ends of chromosomes. Characteristic upstream and downstream sequences adjacent to the coding portions of the genes are given. The sequences UAA and UAG are preferred over CAA and CAG for glutamine while UGA is the true stop codon.

Transformation of Paramecium by microinjection of a cloned serotype gene

Paramecia of a given serotype express only one of several possible surface proteins called immobilization antigens (i-antigens). A 16-kilobase plasmid containing the gene for immobilization antigen A from Paramecium tetraurelia, stock 51, was injected into the macronucleus of deletion mutant d12, which lacks that gene. Approximately 40% of the injected cells acquired the ability to express serotype A at 34 degrees C. Expression appeared to be regulated normally. The transformed cells, like wild type, could be switched to serotype B by antiserum treatment and culture at 19 degrees C; on transfer to 34 degrees C, they switched back to serotype A expression. Many of the lines retained the ability to express serotype A until autogamy, when the old macronucleus is replaced by a new one derived from the micronucleus. DNA from transformants contained the injected plasmid sequences, which were replicated within the paramecia. No evidence for integration was obtained. The majority of replicated plasmid DNA comigrated with a linearized form of the input plasmid. Nonetheless, the pattern of restriction fragments generated by transformant DNA and that generated by input plasmid DNA are identical and consistent with a circular rather than a linear map. These conflicting observations can be reconciled by assuming that a mixture of different linear fragments is present in the transformants, each derived from the circular plasmid by breakage at a different point. Copy-number determinations suggest the presence of 45,000-135,000 copies of the injected plasmid per transformed cell. These results suggest that the injected DNA contains information sufficient for both controlled expression and autonomous replication in Paramecium.

Deviation from the universal code shown by the gene for surface protein 51A in Paramecium

The immobilization antigens (i-antigens) of Paramecium, large polypeptides of relative molecular mass approximately 300,000, are located on the cell surface. Each i-antigen is encoded by a different unlinked gene, and no more than one gene is expressed at a time. The proteins and the mRNAs and genes encoding them are readily isolated. Here we report the nucleotide sequence of three regions of the A i-antigen gene from stock 51 of Paramecium tetraurelia. Surprisingly, all reading frames contain TAA and TAG stop codons, even though there is evidence that one reading frame of these sequences codes for the i-antigen. We suggest that in Paramecium UAA and UAG code for amino acids, instead of serving as translational stops as they do in all other organisms.

Metals in downtown Washington, DC gardens

Soil samples from 95 gardens and leaf vegetables from 33 gardens in an approximately 3 km(2) area of downtown Washington, DC were analyzed for Pb, Cd, Cu, and Zn. The mean soil Pb was 680 μg/g with a range of 40-5300 μg/g. Soil Pb values in this sample were considerably higher than those previously found for a sample of 70 gardens distributed throughout the city. In the downtown sample, 46% of the soil Pb values were above 500 μg/g and 17% were above 1000 μg/g. Soil Pb levels were found to be higher near the house than away from the house, a pattern that was also evident, though less pronounced, for Zn and Cd. Lead-based exterior paint was identified as the most likely source of soil Pb in several cases. The mean leaf vegetable Pb was 6.4 μg/g dry weight, a small but significant difference from the mean value of 4.5 μg/g obtained from the city-wide sample of 38 gardens.

Structure and expression of genes for surface proteins in Paramecium

Surface proteins from 11 antigenic types of Paramecium tetraurelia vary in molecular weight from 251,000 to 308,000. The size of a series of polyadenylated RNAs obtained from these types were correlated with the sizes of the proteins and judged to be the mRNAs for the proteins. The mRNAs were used to identify genomic DNA clones containing complementary sequences. The gene for antigen A was present in one copy per genome, and the data suggest that extensive introns were absent. When restriction enzyme digests of DNA from cultures of paramecia with active and inactive genes were probed with portions of the cloned genes, no evidence for rearrangements or changes in gene dosage was found.

Sample preparation in determination of lead in garden vegetables by flame atomic absorption spectrophotometry

Dry and wet ashing methods have been used in the analysis of garden vegetables for Pb. The reliability of wet ashing has been verified by the method of standard additions. Comparison of dry and wet ashing showed good agreement for a variety of garden vegetables. Sample size was more strictly limited for the wet-ashed samples, which led to lower sensitivity. Vegetable samples are commonly analyzed for a number of trace elements, which introduces additional constraints on sample preparation, notably because of Cd loss on dry ashing. Pretreatment with HNO3/H2SO4 ash aid eliminated Cd loss. Reliability of dry ashing with pretreatment was shown with NBS SRM Orchard Leaves, Pine Needles, Spinach, and Tomato Leaves. The analysis was insensitive to ashing temperature in the range 480-625 degrees C. A practical detection limit for the method is about 2 ppm Pb, dry weight basis (DWB). Care must be exercised to avoid contamination of the sample with lead at this level by improper handling. Segregation and acid washing of glassware and protection of the sample from contact with any object not demonstrably clean was necessary. No evidence was found of Pb contamination at this level from tap water washing of fresh vegetables, forced-air oven drying, or grinding with mortar and pestle. No special clean room facilities or laboratory air purification measures were used. Sensitivity was increased 3-fold by extraction with dithizone in CHCl3 followed by back-extraction into dilute HCl. Detection limits were not improved, however, because of variation in the extraction results. The instrumental method for assessing effective correction for back-ground absorbance showed adequate compensation, although comparison of direct and extractive determinations showed a small but significant difference between the methods of about 1 ppm Pb (DWB).

Quantitative predictions of random segregation models of the ciliate macronucleus

Models of the macronucleus inParamecium tetraureliawhich assume known levels of ploidy and random segregation of subunits smaller than a haploid set at both fission and macronuclear regeneration (MR) are not consistent with the hypothesis that senescence is due to aneuploid imbalance. Either senescence has some other basis or there is a mechanism for regular distribution of subunits at MR. Random segregation models for fission and MR are consistent with most data on survival and heterozygote stability, but if the ploidy level is 860 they fail to account for the data of Nyberg (this volume). Since the ploidy level may be higher than 860, models of random segregation cannot be ruled out forParamecium. Models of the macronucleus in hypotrichs which assume randomly segregating chromosome fragments are consistent with data on survival and on stability of heterozygous genotypes at fission.

Kappa and other endosymbionts in Paramecium aurelia

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The Classes of Kappa in Paramecium Aurelia

Kappas (bacterial symbionts containing R bodies) have been studied in 16 strains of Paramecium aurelia, syngens 2 and 4. All produce toxins capable of killing sensitive paramecia. The first major class, the 51 group (consisting of the kappas of strains 51, 116, and 298), has R bodies which, when the pH is lowered below 6.5, unwind reversibly from the inside to form a tight tube; the outside end of the R-body ribbon forms an acute angle; no sheath surrounds the R body. The phage-like structures of the 51 group are helical; no capsomere-like structures are present; isolated R bodies do not have any killing activity. The second major class, the 7 group (strains 7, 576, Bl 166-1, 249, 1041, 310, 1039), has R bodies which unwind irreversibly from the outside to form a loose twisted ribbon whose outside tip is blunt, irregular or finger-like; a single membraneous sheath covers the R body. Phage-like structures in the 7 group are spherical; capsomere-like structures are present; isolated R bodies show killing activity. The third major class, the 562 group (strains 562, 517, 511), is similar to the 7 group except that there is no sheath, no capsomeres, and free R bodies are almost or completely inactive. The phage-like structures are spherical, and unlike those of the 7 group, do not stick to the R bodies. In addition to these 3 major classes there are 3 strains which show important differences from the major classes and also differ among themselves. 1038 is like 7 in all respects except that the phage-like structures are helical and isolated R bodies are not very active. 51ml, presumably a mutant of 51, has spherical phage-like structures and 7 type-R bodies which are inactive when isolated. 570 is the only known mate-killer with R bodies; it has spherical phage-like structures.

It is noted that strain 1039 of the 7 group has very few phage-like structures, virtually all of which are empty. Since free 1039 R bodies are highly active, it is likely that intact phage-like structures are not essential for the toxic action on sensitives. The spherical and helical phage-like structures are probably very closely related because 51, which has helical structures, apparently gave rise to 51m1, which has spherical structures. Likewise the kappa symbionts of 1038 with helical phage-like structures are virtually identical to the kappas of the 7 group with spherical phage-like structures. The presence of phage-like structures in all strains with R bodies suggests that R bodies may be a product of phage activity. Strain 570, a mate-killer whose symbionts contain R bodies, provides a link between kappas and mus, the mate-killer symbionts. The symbionts of 570 are the only ones containing R bodies which are non-toxic when encountered by sensitives in the medium in which the paramecia are cultured.

The classes of endosymbiont of Paramecium aurelia

The endosymbionts of Paramecium aurelia appear to consist of a number of different Gramnegative bacteria which have come to live within many strains of paramecia. It is not known whether in nature this relationship is mutually beneficial or not. The symbionts from one paramecium may kill other paramecia lacking that kind of symbiont. We identify the following classes of endosymbiotic organisms. First, kappa particles (found in P. aurelia, syngens 2 and 4) ordinarily contain highly characteristic refractile, or R, bodies, which are associated with the production of a toxin which kills sensitive paramecia. In certain mutants of kappa found in the laboratory both R bodies and ability to kill have been lost. Second, mu particles (in syngens 1, 2 and 8) produce the phenomenon of mate-killing. Third, lambda (syngens 4 and 8) and sigma particles (syngen 2) are very large, flagellated organisms which kill only paramecia of syngens 3, 5 and 9, and are enclosed in membrane-bound vacuoles. Fourth, gamma particles (syngen 8) are minute endosymbionts, surrounded by an additional membrane resembling endoplasmic reticulum. They have strong killing activity but no R bodies. Fifth, delta particles (syngens 1 and 6) possess a dense layer covering the outer membrane. At least one of the two known stocks is a killer. Sixth, nu particles are a heterogeneous group of particles (syngens 2 and 5) which do not kill or possess distinctive morphological characteristics. Seventh, alpha particles (syngen 2) are the only known nuclear symbionts of P. aurelia; they are found in the macronucleus. Alpha is also exceptional in being the only particle which is highly infectious, though certain of the other symbionts can also be taken up by paramecia lacking them, under special conditions.

The relation between virus-like particles and R bodies of Paramecium aurelia

Virus-like particles can be seen in sections of kappa of stocks 7 and 562. The virus-like particles are specifically associated with R bodies and are rare or absent in kappa particles lacking R bodies. This observation clearly links the virus-like particles with kappa's toxic activity; it also supports the suggestion that kappa is infected with a lysogenic virus whose induction results in the production of the R body and viruses. Many ‘unfilled’ virus-like particles are present in whole kappa particles. Wound R bodies of stock 7 are surrounded by a sheath, whose breakage may provide the mechanism which causes the unrolling of the R body. One edge of the R body ribbon stains more intensely than the other.

Virus-like bodies in killer paramecia

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