Molecular Structures and Coding Genes of the Water-Borne Protein Pheromones of Euplotes petzi, an Early Diverging Polar Species of Euplotes

Euplotes is diversified into dozens of widely distributed species that produce structurally homologous families of water-borne protein pheromones governing self-/nonself-recognition phenomena. Structures of pheromones and pheromone coding genes have so far been studied from species lying in different positions of the Euplotes phylogenetic tree. We have now cloned the coding genes and determined the NMR molecular structure of four pheromones isolated from Euplotes petzi, a polar species which is phylogenetically distant from previously studied species and forms the deepest branching clade in the tree. The E. petzi pheromone genes have significantly shorter sequences than in other congeners, lack introns, and encode products of only 32 amino acids. Likewise, the three-dimensional structure of the E. petzi pheromones is markedly simpler than the three-helix up-down-up architecture previously determined in another polar species, Euplotes nobilii, and in a temperate-water species, Euplotes raikovi. Although sharing the same up-down-up architecture, it includes only two short α-helices that find their topological counterparts with the second and third helices of the E. raikovi and E. nobilii pheromones. The overall picture that emerges is that the evolution of Euplotes pheromones involves progressive increases in the gene sequence length and in the complexity of the three-dimensional molecular structure.

Evidence for gene duplication and allelic codominance (not hierarchical dominance) at the mating-type locus of the ciliate, Euplotes crassus

The high-multiple mating system of Euplotes crassus is known to be controlled by multiple alleles segregating at a single locus and manifesting relationships of hierarchical dominance, so that heterozygous cells would produce a single mating-type substance (pheromone). In strain L-2D, now known to be homozygous at the mating-type locus, we previously identified two pheromones (Ec-α and Ec-1) characterized by significant variations in their amino acid sequences and structure of their macronuclear coding genes. In this study, pheromones and macronuclear coding genes have been analyzed in strain POR-73 characterized by a heterozygous genotype and strong mating compatibility with L-2D strain. It was found that POR-73 cells contain three distinct pheromone coding genes and, accordingly, secrete three distinct pheromones. One pheromone revealed structural identity in amino acid sequence and macronuclear coding gene to the Ec-α pheromone of L-2D cells. The other two pheromones were shown to be new and were designated Ec-2 and Ec-3 to denote their structural homology with the Ec-1 pheromone of L-2D cells. We interpreted these results as evidence of a phenomenon of gene duplication at the E. crassus mating-type locus, and lack of hierarchical dominance in the expression of the macronuclear pheromone genes in cells with heterozygous genotypes.

Thermodynamic stability of psychrophilic and mesophilic pheromones of the protozoan ciliate euplotes

Three psychrophilic protein pheromones (En-1, En-2 and En-6) from the polar ciliate, Euplotes nobilii, and six mesophilic pheromones (Er-1, Er-2, Er-10, Er-11, Er-22 and Er-23) from the temperate-water sister species, Euplotes raikovi,were studied in aqueous solution for their thermal unfolding and refolding based on the temperature dependence of their circular dichroism (CD) spectra. The three psychrophilic proteins showed thermal unfolding with mid points in the temperature range 55–70 °C. In contrast, no unfolding was observed for any of the six mesophilic proteins and their regular secondary structures were maintained up to 95 °C. Possible causes of these differences are discussed based on comparisons of the NMR structures of the nine proteins.

Detection of a novel subspecies of Francisella noatunensis as endosymbiont of the ciliate Euplotes raikovi

Francisella are facultative intracellular bacteria causing severe disease in a broad range of animals. Two species are notable: Francisella tularensis, the causative organism of tularemia and a putative warfare agent, and Francisella noatunensis, an emerging fish pathogen causing significant losses in wild and farmed fish. Although various aspects of Francisella biology have been intensively studied, their natural reservoir in periods between massive outbreaks remains mysterious. Protists have been suspected to serve as a disguised vector of Francisella and co-culturing attempts demonstrate that some species are able to survive and multiply within protozoan cells. Here, we report the first finding of a natural occurrence of Francisella sp. as a protist endosymbiont. By molecular and morphological approaches, we identified intracellular bacteria localized in a strain of the marine ciliate Euplotes raikovi, isolated from the coast of Adriatic Sea. Phylogenetic analysis placed these endosymbionts within the genus Francisella, in close but distinct association with F. noatunensis. We suggest the establishment of a novel subspecies within F. noatunensis and propose the cytoplasmatic endosymbiont of E. raikovi as "Candidatus F. noatunensis subsp. endociliophora" subsp. nov.

Characterization of the pheromone gene family of an Antarctic and Arctic protozoan ciliate, Euplotes nobilii

Allelic genes encoding water-borne signal proteins (pheromones) were amplified and sequenced from the somatic (macronuclear) sub-chromosomic genome of Antarctic and Arctic strains of the marine ciliate, Euplotes nobilii. Their open reading frames appeared to be specific for polypeptide sequences of 83 to 94 amino acids identifiable with cytoplasmic pheromone precursors (pre-pro-pheromones), requiring two proteolytic steps to remove the pre- and pro-segments and secrete the mature pheromones. Differently from most of the macronuclear genes that have so far been characterized from Euplotes and other hypotrich ciliates, the 5' and 3' non-coding regions of all the seven E. nobilii pheromone genes are much longer than the coding regions (621 to 700 versus 214 to 285 nucleotides), and the 5' regions in particular show nearly identical sequences across the whole set of pheromone genes. These structural peculiarities of the non-coding regions are likely due to the presence of intron sequences and provide presumptive evidence that they are site of basic, conserved activities in the mechanism that regulates the expression of the E. nobilii pheromone genes.