The genome of the sea anemone is not as simple as we thought. The complexity of its regulatory elements is similar to that of fruit flies and other animals, suggesting the principles of gene regulation -- the way various genes interact, turning some on and others off -- is 600 million years old and dates back to our common ancestor.
In the face of this newfound complexity, researchers have also discovered that sea anemone are more similar to plants than animals when it comes to the particular way they regulate their genes.
Genes rarely work alone. If only it were so simple. Instead, they act in concert, regulating each other's activities and expression in ‘gene regulatory networks.’ The absence or presence of an individual gene can’t, for example, explain how you look. Organisms with simple body plans, like sea anemone, have the complex gene repertoire of ‘higher’ animal models. But how do you measure the complexity of gene regulation? Some researchers suggest looking at the distribution and density of regulatory sequences in the genome.
A quick glossary of terms, in case you want to brush up. Gene expression is when the information on a ‘target’ gene is used to create a functional gene product (usually proteins). That’s when double-stranded DNA unzips and single-stranded RNA is synthesized on that DNA template -- that’s called transcription. Transcription factors, then, are the regulatory proteins that bind to DNA to stimulate transcription of specific genes. Regulatory sequences of DNA -- motifs called enhancers and promoters -- bind to transcription factors to regulate the expression of target genes at a specific time and place. (If genes make up the words in the language of genetics, enhancers and promoters serve as the grammar.)
Using molecular methods, Ulrich Technau of the University of Vienna and an international team of collaborators were able to identify promoters and enhancers on a genome-wide level in the sea anemone. When they compared the data to higher organisms, they found that sea anemone gene regulation was similarly complex.
In addition to the control of transcription of DNA to RNA, the expression of a gene can also be regulated after transcription; this post-transcriptional level takes place after the RNA is already produced. Here’s where microRNAs play an important role. These short regulatory RNAs bind to target RNAs to inhibit their actions and gene expression, ultimately affecting metabolism and various developmental processes; their mutations are associated diseases like cancer. According to Technau, up to 50 percent of all human genes are regulated by microRNAs, and over a thousand have been identified in humans and animals in just the last few years.
But the evolutionary origin of animal microRNAs is still unclear, and while they've also been discovered in plants, scientists assumed that they arose independently from animal microRNAs.
So, the team managed to isolate 87 microRNAs from the sea anemone. They were surprised to find that their microRNAs depict all the hallmarks of plant microRNAs. Additionally, a particular gene that's essential for microRNA in plants was never detected in any other animal model organism before, until now. And when they compared the sequences of microRNAs, one with similarity to a plant microRNA, as well as one with similarity to an animal microRNA can be found.
Together, these findings suggest the first evolutionary link between microRNAs of plants and animals. So while the sea anemone's genome, gene repertoire, and gene regulation on the DNA level is surprisingly similar to vertebrates, its post-transcriptional regulation is undeniably plant-like -- and probably dates back 600 million years to the common ancestor of animals and plants.
[Via University of Vienna]
Image: Nematostella vectensis / Nature, 2005