Genome complexity: RNA gene activity controls (Introduction)

by David Turell , Tuesday, July 28, 2020, 19:27 (1366 days ago) @ David Turell

RNA can stop expression of genes:

https://phys.org/news/2020-07-amazing-small-rnas.html

"In most organisms, small bits of RNA play a key role in gene regulation by silencing gene expression. They do this by targeting and docking onto complementary sequences of gene transcripts (also RNA molecules), which stop the cell machinery from using them to make proteins. This mechanism is called RNA interference (RNAi), and it is critically important in biology.

"Remarkably, the RNAi phenomenon is not necessarily confined to single cells; it can also manifest in other tissues and organs far away from the cell of origin. Researchers have been able to observe it mostly in plants, but also in 'lower' animals such as the nematode worm C. elegans.

***

"They are the first to demonstrate unequivocally that these distant messengers in plants are short double-stranded RNA molecules. These consist of pairs (or double-strands) of just 21 to 24 nucleotides (the building blocks of RNA) called small interfering RNAs, or siRNAs for short. The team's paper was recently published in the journal Nature Plants.

"siRNAs usually emerge as large and complex populations from the genomes of viruses that have infected a cell. But a cell's own genes can also serve as blueprints for these molecules. As a result, cells can use RNAi to silence not only invading viruses but also their own genes.

***

"Not only did the ETH researchers identify the elusive long-distance messengers, they also show, in their study, how siRNAs move and carry out their function. They found that, as long as an siRNA molecule exists as a free double-strand, it is mobile because it cannot bind to a matching RNA transcript. To bind, it first has to be "uploaded" to a specific Argonaute (AGO) effector protein. Only once bound to the correct AGO protein can the siRNA silence the target transcript; the process eventually destroys the fragment itself. The model plant used for the study has ten different AGO proteins, several of which recognize matching siRNA fragments with specific signatures; these signatures are not homogeneous among the large cohorts of mobile siRNAs produced from viruses or the plant's own genes.

***

"'The amount and diversity of AGO proteins in traversed cells coupled to the siRNA-intrinsic signatures function together as a kind of molecular sieve, the form of which may differ from cell type to cell type along the siRNA path. Depending on the spatial configuration of this sieve, a wide variety of siRNA movement patterns can be produced," Voinnet explains. He adds, "Even more interestingly, some AGOs can be induced by stress or developmental signals such that the spatial shape of the sieve can change and evolve at any given time".

"The countless movement patterns thus lend the mobile RNAi system almost boundless flexibility and versatility in shaping gene expression across distances. Now that they have understood the process, the team of researchers is trying to engineer artificial sieves in plants as a way to control, with high precision, when and where specific siRNAs can move, a method which could have applications in agriculture."

Comment: Certainly another layer of genetic controls. Perhaps it is the same in humans, where so few genes produce so complex an organism.