TAPAN KUMAR MAITRA EXPLAINS RNA INVOLVEMENT IN SILENCING THE EXPRESSION OF GENES CONTAINING COMPLEMENTARY BASE SEQUENCES

Regulatory proteins that bind to specific mRNAs — as is the case with the IRE-binding protein — are not the only molecules used by cells to control mRNA activity. Individual mRNAs can also be controlled by a special class of short RNA molecules that inhibit the expression of those mRNAs that contain sequences related to that of the short RNAs. Such RNA-mediated inhibition, known as RNA interference, is based on the ability of short RNAs to trigger mRNA degradation, or inhibit mRNA translation, or inhibit transcription of the gene coding for a particular mRNA.
   The first type of RNA interference to be discovered occurs as a response to the introduction of double-stranded RNA. For example, if plants are infected with viruses that produce double-stranded RNA as part of their life cycle, the RNA interference mechanism shuts down expression of the viral genes and thereby limits viral infection. Moreover, the effect is not limited to viral genes. If a virus is genetically engineered to contain a normal plant gene, cells infected with the virus shut down expression of their own normal copy of the same gene.
   The mechanism that allows a double-stranded RNA to silence the expression of specific genes is illustrated. First, a ribonuclease known as Dicer cleaves the double-stranded RNA into short fragments about 21-22 base pairs in length. The resulting double-stranded fragments, called siRNAs (small interfering RNAs), are then combined with a group of proteins to form a complex known as RISC (RNA-induced silencing complex). After being incorporated into a RISC, one of the two strands of the siRNA is degraded. The remaining single-stranded RNA then binds the RISC via complementary base pairing to a target mRNA molecule.
   If pairing between the siRNA and the mRNA is a perfect match (or very close), the mRNA is degraded by Slicer, a ribonuclease component of the RISC that cleaves the mRNA in the middle of the complementary site. If the match between the siRNA and mRNA is imperfect, translation of the mRNA may be inhibited without the mRNA being degraded. And, in some cases, the RISC may enter the nucleus and be guided by its siRNA to complementary nuclear DNA sequences. After associating with these gene sequences, the RISC silences their expression by stimulating DNA methylation and/or recruiting an enzyme that adds methyl groups to histones, thereby triggering the formation of a transcriptionally inactive, condensed form of chromatin (heterochromatin).
   RNA interference may have originally evolved to protect cells from viruses that utilise double-stranded RNA. However, it also turns out to be a powerful laboratory tool that allows scientists to selectively shut down any gene they wish to study. Since complete genome sequences are now available for a variety of organisms, the function of each individual gene can be systematically explored by using RNA interference to turn it off.
   Researchers simply synthesise (or purchase) short siRNAs that are complementary to sequences present in the genes they wish to silence. Introducing these synthetic siRNAs into cells allows individual genes to be turned off one at a time. To illustrate the extraordinary power of this approach, synthetic siRNAs have already been used to individually turn off almost all of the 19,000 genes in the worm C. elegans.

THE WRITER IS ASSOCIATE PROFESSOR, HEAD, DEPARTMENT OF BOTANY, ANANDA MOHAN COLLEGE, KOLKATA, AND ALSO FELLOW, BOTANICAL SOCIETY OF BENGAL, AND CAN BE CONTACTED AT tapanmaitra59@yahoo.co.in