In protozoan ciliates, diffusible signalling proteins (pheromones) regulate the vegetative growth and mating interactions. Here, the coding genes and the structures of the encoded pheromones were studied in genetically distinct wild-type strains representing interbreeding Antarctic and Arctic populations of the marine ciliate Euplotes nobilii. Determination of seven allelic pheromone-coding DNA sequences revealed that an unusual extension and high structural conservation of the 5' non-coding region are peculiar traits of this gene family, implying that this region is directly involved in the mechanism of pheromone gene expression, possibly through phenomena of intron splicing and/or frame-shifting. For four pheromones, the three-dimensional structures were determined by nuclear magnetic resonance spectroscopy in solution. These structures show that the pheromones represent a protein family which adapts to its polar environment by combining a structurally stable core of a three-helix bundle with extended polypeptide segments that are devoid of regular secondary structures and concomitantly show enhanced structural flexibility.
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