14 DAY TRIAL // These are the findings of MIT, the Whitehead Institute for Biomedical Research and the Institute for Systems Biology in Seattle. An American team has discovered the function played by an enigmatic RNA type which proved to have an important function in protecting sex cells from self-destructing by mitotic division. This function is linked to the processes of gene expression.
When a gene has started the process of protein synthesis, the two strands of the double DNA molecule that comprise the gene unravel. The first strand molds a molecule of messenger RNA, which is the protein's template, called the "sense" or "coding" transcript. The other DNA strand doesn't provide a protein recipe, but on occasion, it produces an "antisense" RNA molecule, whose sequence is complementary to that of the "sense" RNA. Even though antisense RNA had been discovered for some genes, its precise function has eluded the scientists for a long time.
Cintia Hongay, a postdoctoral researcher at the Whitehead Institute for Biomedical Research and MIT Biology Professor Gerald Fink have found that, in the case of a gene from common yeast, called IME4, the antisense RNA blocks the sense RNA. In other words, the gene disables its own ability to make protein. "This is the first case where a specific function in a higher cell for antisense RNA has been found," said Fink. "This points to an entirely new process of gene regulation that we've never seen before in eukaryotic cells."
When the yeast encounters favorable food and environmental conditions, its cells divide by mitosis: the DNA amounts duplicate and it results a double number of chromosomes, grouped in identical pairs, and each daughter cell receives exactly the same number of chromosomes as the original cell, one from every perfectly similar couples of chromosomes. When the conditions worsen, IME4 activates itself starting a cell division type called meiosis.
The mother cell put into the two resulting germ-cells, exactly like in animal egg and sperm cells, half of the number of chromosomes, as the chromosome number was double, each chromosome having a correspondent. Yeast germs (also called spores) are much more resistant to harsh environment than the larger mother cells.
But meiosis can have place just once, and, if the process is not stopped, haploid cells (with only one chromosome from a correspondent pair) will produce inviable progeny, as every cell will lack essential genes. But potential destructive meiosis is avoided in haploid cells as they continually produce IME4 antisense RNA, which stop the production of sense RNA, stopping meiotic division. "For years scientists have evaluated genomes by measuring the sense RNA, with antisense transcripts thought to have no meaning at all," says Fink.
"Here we've found a process in which antisense RNA regulates sense RNA. This same process may occur in the sex cells of mammals. In fact, considering how widespread these antisense transcripts are, I wouldn't be surprised if these findings eventually lead us to discover an entirely new level of gene regulation."
The team is now on the search of another antisense RNA regulated genes in the yeast genome.