Ciliates: mixing things up

Ciliates are mostly unicellular protists that can be found in a variety of environments, ranging from marine and freshwater habitats to animal intestines. Their genomes defy many of the rules found in introductory textbooks. One rule many ciliate species don’t follow is using the “universal” genetic code. The four letters of DNA—A, T, C, and G—code for amino acids that build proteins in triplets called codons. The codon sequence ATG, for example, codes for the amino acid methionine. But three codons typically code for stop sequences that trigger the end of protein synthesis, such as the sequence TAA. In nearly all other living things, this sequence (TAA) is a stop signal, but in many ciliates, like Paramecium bursaria, this sequence codes for the amino acid glutamine instead. A recent study even suggests that one ciliate species, Condylostoma magnum, has a genetic code where all three stop sequences have been re-coded!

Perhaps the most remarkable feature of ciliates though is they possess two different types of nuclei, a trait called nuclear dimorphism. These two nuclei are both functionally and structurally distinct. The larger macronucleus controls the day to day functioning of the cell while the smaller micronucleus is strictly used for sexual reproduction. The genome of the macronucleus is derived from that of the micronucleus but looks dramatically different. For example, the micronucleus of the model ciliate Tetrahymena thermophila contains five chromosomes, but its macronucleus contains approximately 225 chromosomes. This striking difference in structure is because the macronuclear genome goes through major rearrangements during its transition from a micronucleus; multiple processes occur, such as editing out about one-third of the genome and making numerous copies of other parts.

The genomic gymnastics of ciliates have led to several major discoveries about genome biology and two Nobel prizes. The ends of chromosomes in eukaryotes have protective caps called telomeres, which are known to play important roles in health—including cancer and aging. The enzyme telomerase, which constructs telomeres, was first discovered and characterized in Tetrahymena. Genomic rearrangement is also a common occurrence in cancer cells, so studying how extreme genome rearrangements occur in ciliates may provide insight in their role in cancer as well. By showing us that the genetic code and genome structure is highly malleable, ciliates can help us understand that the language of life is not set in stone.