Isolation and composition of bacteriophage-like particles from kappa of killer Paramecia

Activation of an Aquareovirus, Chum Salmon Reovirus (CSV), by the Ciliates Tetrahymena thermophila and T. canadensis

For the first time, ciliates have been found to activate rather than inactivate a virus, chum salmon reovirus (CSV). Activation was seen as an increase in viral titre upon incubation of CSV at 22 °C with Tetrahymena canadenesis and two strains of T. thermophila: wild type (B1975) and a temperature conditional mutant for phagocytosis (NP1). The titre increase was not likely due to replication because CSV had no visible effects on the ciliates and no vertebrate virus has ever been shown unequivocally to replicate in ciliates. When incubated with B1975 and NP1 at 30 °C, CSV was activated only by B1975. Therefore, activation required CSV internalization because at 30 °C only B1975 exhibited phagocytosis. CSV replicated in fish cells at 18 to 26 °C but not at 30 °C. Collectively, these observations point to CSV activation being distinct from replication. Activation is attributed to the CSV capsid being modified in the ciliate phagosomal-lysosomal system and released in a more infectious form. When allowed to swim in CSV-infected fish cell cultures, collected, washed, and transferred to uninfected cultures, T. canadensis caused a CSV infection. Overall the results suggest that ciliates could have roles in the environmental dissemination of some fish viral diseases.

The genus Caedibacter comprises endosymbionts of Paramecium spp. related to the Rickettsiales (Alphaproteobacteria) and to Francisella tularensis (Gammaproteobacteria)

Obligate bacterial endosymbionts of paramecia able to form refractile inclusion bodies (R bodies), thereby conferring a killer trait upon their ciliate hosts, have traditionally been grouped into the genus CAEDIBACTER: Of the six species described to date, only the Paramecium caudatum symbiont Caedibacter caryophilus has been phylogenetically characterized by its 16S rRNA gene sequence, and it was found to be a member of the Alphaproteobacteria related to the RICKETTSIALES: In this study, the Caedibacter taeniospiralis type strain, an R-body-producing cytoplasmatic symbiont of Paramecium tetraurelia strain 51k, was investigated by comparative 16S rRNA sequence analysis and fluorescence in situ hybridization with specific oligonucleotide probes. C. taeniospiralis is not closely related to C. caryophilus (80% 16S rRNA sequence similarity) but forms a novel evolutionary lineage within the Gammaproteobacteria with the family Francisellaceae as a sister group (87% 16S rRNA sequence similarity). These findings demonstrate that the genus Caedibacter is polyphyletic and comprises at least two phylogenetically different bacterial species belonging to two different classes of the PROTEOBACTERIA: Comparative phylogenetic analysis of C. caryophilus, five closely related Acanthamoeba endosymbionts (including one previously uncharacterized amoebal symbiont identified in this study), and their hosts suggests that the progenitor of the alphaproteobacterial C. caryophilus lived within acanthamoebae prior to the infection of paramecia.

R-body-producing bacteria

Until 10 years ago, R bodies were known only as diagnostic features by which endosymbionts of paramecia were identified as kappa particles. They were thought to be limited to the cytoplasm of two species in the Paramecium aurelia species complex. Now, R bodies have been found in free-living bacteria and other Paramecium species. The organisms now known to form R bodies include the cytoplasmic kappa endosymbionts of P. biaurelia and P. tetraurelia, the macronuclear kappa endosymbionts of P. caudatum, Pseudomonas avenae (a free-living plant pathogen), Pseudomonas taeniospiralis (a hydrogen-oxidizing soil microorganism), Rhodospirillum centenum (a photosynthetic bacterium), and a soil bacterium, EPS-5028, which is probably a pseudomonad. R bodies themselves fall into five distinct groups, distinguished by size, the morphology of the R-body ribbons, and the unrolling behavior of wound R bodies. In recent years, the inherent difficulties in studying the organization and assembly of R bodies by the obligate endosymbiont kappa, have been alleviated by cloning and expressing genetic determinants for these R bodies (type 51) in Escherichia coli. Type 51 R-body synthesis requires three low-molecular-mass polypeptides. One of these is modified posttranslationally, giving rise to 12 polypeptide species, which are the major structural subunits of the R body. R bodies are encoded in kappa species by extrachromosomal elements. Type 51 R bodies, produced in Caedibacter taeniospiralis, are encoded by a plasmid, whereas bacteriophage genomes probably control R-body synthesis in other kappa species. However, there is no evidence that either bacteriophages or plasmids are present in P. avenae or P. taeniospiralis. No sequence homology was detected between type 51 R-body-encoding DNA and DNA from any R-body-producing species, except C. varicaedens 1038. The evolutionary relatedness of different types of R bodies remains unknown.

Extrachromosomal elements of extrachromosomal elements of Paramecium and their extrachromosomal elements

Expression of killer traits in Paramecium is due to a complex interaction between the lower eukaryote host and two or three elements that can be viewed either as extrachromosomal elements or as endosymbionts. In all cases, the determinants of the killer trait are carried by obligate bacterial endosymbionts belonging to the genus Caedibacter. However, the actual genetic determinants for expression of these traits are not an integral part of the symbiont genome. They are located on extrachromosomal genetic elements (plasmids or bacteriophages) which essentially are molecular endosymbionts of Caedibacter. In the case of the plasmids, they are associated with yet another set of extrachromosomal genetic elements, which are transposons. These transposons have been observed to move into new sites in the plasmids and even to disrupt expression of R body production and the killer trait. Thus, the transposons can be considered either as extrachromosomal elements of extrachromosomal elements (plasmids) of extrachromosomal elements (C. taeniospiralis) of paramecia, or as molecular parasites of molecular endosymbionts (plasmids) of bacterial endosymbionts of paramecia.

Physical map of a plasmid from Caedibacter taeniospiralis 51

Caedibacter taeniospiralis 51 carries at least two plasmids, pKAP51 and pKAP52. The smaller plasmid, pKAP51, contains 43 kilobase pairs. The larger plasmid, pKAP52, contains more than 110 kilobase pairs. Relative positions of recognition sequences for seven different restriction endonucleases were determined, and a physical map of pKAP51, consisting of a total of 28 restriction sites, was constructed.

Covalently closed, circular DNA in kappa endosymbionts of Paramecium

Caedobacter taeniospiralis(kappa), a bacterial endosymbiont isolated fromParamecium tetraureliastock 51, contains, in addition to the bacterial chromosome, covalently closed circular DNA molecules as shown by isolation on dye-buoyant-density gradients. The closed circular molecule has a contour length of 13·75 ± 0·04 µm with a buoyant density of 1·698 g/cm3. The buoyant density of the bacterial chromosome is 1·700–1·701 g/cm3. Kappa of the 51 group isolated from stock 298 and stock 6g2,P. tetraurelia, also contain the closed circular DNA. Two forms of kappa coexist in paramecia: brights and nonbrights. Examination by density-gradient centrifugation of the DNA of brights and nonbrights shows the extrachromosomal DNA to be associated mainly with brights. It is suggested that the extrachromosomal DNA might be the determinant for the refractile bodies and the helical phage-like structures found in brights.

Kappa and other endosymbionts in Paramecium aurelia

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