The pheromones and pheromone genes of new stocks of the Euplotes octocarinatus species complex

Primary Structure and Coding Genes of Two Pheromones from the Antarctic Psychrophilic Ciliate, Euplotes focardii

In ciliates, diffusible cell type-specific pheromones regulate cell growth and mating phenomena acting competitively in both autocrine and heterologous fashion. In Euplotes species, these signaling molecules are represented by species-specific families of structurally homologous small, disulfide-rich proteins, each specified by one of a series of multiple alleles that are inherited without relationships of dominance at the mat-genetic locus of the germinal micronuclear genome, and expressed as individual gene-sized molecules in the somatic macronuclear genome. Here we report the 85-amino acid sequences and the full-length macronuclear nucleotide coding sequences of two pheromones, designated Ef-1 and Ef-2, isolated from the supernatant of a wild-type strain of a psychrophilic species of Euplotes, E. focardii, endemic to Antarctic coastal waters. An overall comparison of the determined E. focardii pheromone and pheromone-gene structures with their homologs from congeneric species provides an initial picture of how an evolutionary increase in the complexity of these structures accompanies Euplotes speciation.

Bioactive Molecules from Ciliates: Structure, Activity, and Applicative Potential

Ciliates are a rich source of molecules synthesized to socialize, compete ecologically, and interact with prey and predators. Their isolation from laboratory cultures is often straightforward, permitting the study of their mechanisms of action and their assessment for applied research. This review focuses on three classes of these bioactive molecules: (i) water‐borne, cysteine‐rich proteins that are used as signaling pheromones in self/nonself recognition phenomena; (ii) cell membrane‐associated lipophilic terpenoids that are used in interspecies competitions for habitat colonization; (iii) cortical granule‐associated molecules of various chemical nature that primarily serve offence/defense functions.

The Sub-Chromosomic Macronuclear Pheromone Genes of the Ciliate Euplotes raikovi: Comparative Structural Analysis and Insights into the Mechanism of Expression

In Euplotes raikovi, we have determined the full-length sequences of a family of macronuclear genes that are the transcriptionally active versions of codominant alleles inherited at the mating-type (mat) locus of the micronuclear genome, and encode cell type-distinctive signaling pheromones. These genes include a 225-231-bp coding region flanked by a conserved 544-bp 5'-leader region and a more variable 3'-trailer region. Two transcription initiation start sites and two polyadenylation sites associated with nonconventional signals cooperate with a splicing phenomenon of a 326-bp intron residing in the 5'-leader region in the generation of multiple transcripts from the same gene. In two of them, the synthesis of functional products depends on the reassignment to a sense codon, or readthrough of a strictly conserved leaky UAG stop codon. That this reassignment may take place is suggested by the position this codon occupies in the transcripts, close to the transcript extremity and far from the poly(A) tail. In such a case, one product is a 69-amino acid protein in search of function and the second product is a 126-amino acid protein that represents a membrane-bound pheromone isoform candidate to function as a cell type-specific binding site (receptor) of the soluble pheromones.

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.

Revisiting fifty years of research on pheromone signaling in ciliates

Among protists, pheromones have been identified in a great variety of algal species for their activity in driving gamete-gamete interactions for fertilization. Analogously in ciliates, pheromones have been identified for their activity in inducing the sexual phenomenon of conjugation. Although this identification was pioneered by Kimball more than fifty years ago, an effective isolation and chemical characterization of ciliate pheromones has remained confined to species of Blepharisma, Dileptus and Euplotes. In Euplotes species, in which the molecular structures have been determined, pheromones form species-specific families of structurally homologous helical, cysteine-rich, highly-stable proteins. Being structurally homologous, they can bind cells in competition with one another, raising interesting functional analogies with the families of growth factors and cytokines that regulate cell differentiation and development in higher organisms. In addition to inducing conjugation by binding cells in heterologous fashion, Euplotes pheromones act also as autocrine growth factors by binding to, and promoting the vegetative reproduction of the same cells from which they originate. This autocrine activity is most likely primary, providing a concrete example of how the original function of a molecule can be obscured during evolution by the acquisition of a new one.

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.

Methionine sulfoxide reduction in ciliates: characterization of the ready-to-use methionine sulfoxide-R-reductase genes in Euplotes

Genes encoding the enzyme methionine sulfoxide reductase type B, specific to the reduction of the oxidized methionine-R form, were characterized from the expressed (macronuclear) genome of two ecologically separate marine species of Euplotes, i.e. temperate water E. raikovi and polar water E. nobilii. Both species were found to contain a single msrB gene with a very simple structural organization encoding a protein of 127 (E. raikovi) or 126 (E. nobilii) amino acid residues that belongs to the group of zinc-containing enzymes. Both msrB genes are constitutively expressed, suggesting that the MsrB enzyme plays an essential role in repairing oxidative damages that appear to be primarily caused by physiological cell aging in E. raikovi and by interactions with an O(2) saturated environment in E. nobilii.

Isolation and structural characterization of two water-borne pheromones from Euplotes crassus, a ciliate commonly known to carry membrane-bound pheromones

Ciliates comprise species synthesizing water-diffusible mating type factors or pheromones and species synthesizing insoluble, cell membrane-bound pheromones. Euplotes crassus has traditionally been placed in the latter group. In contrast with this notion, we found that E. crassus is a constitutive pheromone-secreting ciliate, like other Euplotes species. From cell-free filtrate preparations of the E. crassus strain L-2D, we isolated two distinct pheromones, designated as Ec-α and Ec-1, and determined their complete amino acid sequences by combined chemical and genetic approaches. The Ec-α pheromone sequence extends for 56 amino acid residues with six cysteines and shows a molecular mass of 6,183 Da, while the Ec-1 pheromone sequence extends for 45 amino acid residues with 10 cysteines and shows a molecular mass of 4,840 Da. Marked structural differences distinguish the full-length Ec-α and Ec-1 coding sequences, which have been cloned and characterized from the transcriptionally active macronuclear genome. They were taken as clear indication that the Ec-α and Ec-1 pheromones are specified by genes that are not allelic, but likely derived from a duplicated genetic locus of the transcriptionally silent micronuclear genome.

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.

The water-borne protein signals (pheromones) of the Antarctic ciliated protozoan Euplotes nobilii: structure of the gene coding for the En-6 pheromoneThis article is one of a selection of papers in the Special Issue on Polar and Alpine Microbiology

The marine Antarctic ciliate, Euplotes nobilii , secretes a family of water-borne signal proteins, denoted as pheromones, which control vegetative proliferation and mating in the cell. Based on the knowledge of the amino acid sequences of a set of these pheromones isolated from the culture supernatant of wild-type strains, we designed probes to identify their encoding genes in the cell somatic nucleus (macronucleus). The full-length gene of the pheromone En-6 was determined and found to contain an open-reading frame specific for the synthesis of the En-6 cytoplasmic precursor (pre-pro-En-6), which requires 2 proteolytic cleavages to remove the signal peptide (pre) and the prosegment before secretion of the mature protein. In contrast to the sequence variability that distinguishes the secreted pheromones, the pre- and pro-sequences appear to be tightly conserved and useful for the construction of probes to clone every other E. nobilii pheromone gene. Potential intron sequences in the coding region of the En-6 gene imply the synthesis of more En-6 isoforms.

Structural characterization of En-1, a cold-adapted protein pheromone isolated from the Antarctic ciliate Euplotes nobilii

The second of two diffusible cell signal proteins (pheromones) purified from a wild-type strain of the Antarctic ciliate, Euplotes nobilii, has been determined by automated Edman degradation of the whole molecule and peptides generated by its chymotryptic digestion. The proposed sequence of 52 amino acids of this new pheromone, designated En-1, is: NPEDWFTPDT(10)CAYGDSNTAW(20)TTCTTPGQTC(30)YTCCSSCFDV(40)VGEQACQMSA(50)QC. In common with the previously determined 60-amino-acid sequence of the other pheromone, En-2, it bears eight cysteines in conserved positions (presumably linked into four conserved intrachain disulfide bonds), and physicochemical features of potential significance for cold adaptation, such as a reduced hydrophobicity, an increased solvent accessibility, and an improved local backbone flexibility. However, En-1 diverges from En-2 for having evolved a threonine cluster in the place of a glycine cluster to apparently make more flexible a region that is likely functionally important.

Structural characterization of a protein pheromone from a cold-adapted (Antarctic) single-cell eukaryote, the ciliate Euplotes nobilii

Free-living species of ciliated Protozoa control their vegetative (mitotic) proliferation and mating (sexual) processes by diffusible, cell type-specific protein signals (pheromones). One of these molecules, designated En-2, was isolated from a species, Euplotes nobilii, living in the stably cold marine waters of Antarctica, and its complete amino acid sequence of 60 residues was determined by automated Edman degradation of the whole protein and peptides generated by trypsin digestion. The proposed sequence is: DIEDFYTSETCPYKNDSQLA(20)WDTCSGGTGNCGTVCCGQCF(40)SFPVSQSCAGMADSNDCPNA(60). The En-2 structure appears to be characterized by an adaptive insertion of a glycine-rich motif potentially capable to confer more flexibility to a functionally critical region of the molecule.

A structurally deviant member of the Euplotes raikovi pheromone family: Er-23

Pheromones of Euplotes raikovi form a homologous family of proteins with 37- to 40-amino acid residues, including six cysteines that form three strictly conserved disulfide bridges. The determination of the primary structure of the pheromone Er-23, which was isolated from cells derived from natural populations of E. raikovi that secrete the other known pheromones, has now revealed a novel structure type. The polypeptide chain of this pheromone contains 51 residues, 10 of which are cysteines presumably involved in the formation of five disulfide bridges, and lacks a carboxyl-terminal tail following the last cysteine of the sequence. The elongation of the Er-23 molecule is presumed to result from multiple events of gene duplication starting from an ancestral motif Xxx(2-4)-Cys-Xxx(5-7)-Cys.

Odd mating-type substances may work as precursor molecules of even mating-type substances in Paramecium caudatum

Mating-type substances are key molecules in the sexual recognition of the odd (O) and even (E) complementary mating-type cells in Paramecium caudatum. Indirect evidence suggested that the substances were proteins and were located on ventral surface cilia. Monoclonal antibodies inhibiting the mating reactivity of the O cells have been obtained. Using these antibodies, we tried to detect antigen molecules as dot-blot signals. Strong dot-blot signals of antigens were only detected from the mating reactive cells, but they were not detected from the well-fed and starved cells without mating reactivity. In addition to identifying the antigen on cilia and cytoplasm of the O cell, the antigen was detected from the cytoplasm of the E cells but never from their cilia. Furthermore, extracts of the E cells induced mating reaction with the living E cells but not with O cells. Thus, the O mating-type substances exist in the cytoplasm of the E mating-type cells, supporting strongly the hypothesis that O mating-type substances are precursor molecules of the E mating-type substances.