Ribosomal rna

Ribosomal RNA

Ribosomal ribonucleic acid (rRNA) is the RNA component of ribosomes, the molecular machines that catalyze protein synthesis. Ribosomal RNA constitute over sixty percent of the ribosome by weight and are crucial for all its functions – from binding to mRNA and recruiting tRNA to catalyzing the formation of a peptide bond between two amino acids. Even the structure of a ribosome is determined by the three-dimensional shape of its rRNA core. Proteins present in the ribosome serve to stabilize this structure through interactions with the core.

Discovery of Ribosomal RNA

Ribosomal RNA was discovered during cell fractionation experiments investigating the role of RNA viruses in causing cancer. Fractionation is a method where cell membranes are carefully and selectively removed while keeping the function of cellular organelles intact. This homogenized cytoplasm is then centrifuged at increasing speeds so that organelles separate according to density. The initial experiments that revealed the presence of rRNA extracted a fraction that was thought to represent a new sub-cellular organelle, microsome, specializing in protein synthesis. Later, it was seen that it was the presence of ribosomes on endoplasmic reticulum that led to the detection of RNA in these samples.

Types of Ribosomal RNA

Both prokaryotic and eukaryotic ribosomes are made of a larger and smaller subunit and these two units come together during mRNA translation. The smaller subunit in prokaryotes is made of an RNA molecule about 1500 nucleotides in length with a Svedberg coefficient of 16S. Together with ribosomal proteins, the smaller subunit has a sedimentation rate of 30S. This is paired with the larger subunit, having two RNA molecules – one that is nearly 3000 nucleotides (23S) in length and the other is a short sequence of 120 nucleotides (5S). These RNA molecules are accompanied by proteins that give rise to the larger 50S subunit.

Functions of Ribosomal RNA

The primary function of rRNA is in protein synthesis – in binding to messenger RNA and transfer RNA to ensure that the codon sequence of the mRNA is translated accurately into amino acid sequence in proteins. To achieve this, rRNA has a distinctive three-dimensional shape involving internal loops and helices that creates specific sites within the ribosome – the A, P and E sites. The P site is for binding a growing polypeptide, the A site anchors an incoming tRNA charged with an amino acid. After peptide bond formation, the tRNA binds briefly to the E site before leaving the ribosome. In addition rRNA also has sites for binding to some ribosomal proteins and careful analysis has demarcated the exact residues in both the RNA and protein.

Role of Ribosomal RNA in Translation

• Messenger RNAs carry the genetic information coded in the DNA into the cytoplasm where the nucleotide sequence is read by ribosomes in stretches of three bases called codons. Four nucleotides, Adenine, Uracil, Guanine and Cytosine, can be arranged to form a total of sixty-four triplet codons. Each codon corresponds to a single amino acid and thus codes for the protein sequence.
• Prokaryotic translation begins with the 16S rRNA base pairing with the Shine-Dalgarno consensus sequence in mRNA. Since the Shine-Dalgarno sequence is 6-10 nucleotides upstream of the start codon, binding with rRNA allows the start codon to be positioned within the ribosome. This interaction is mediated by other proteins, which also recruit the larger ribosomal subunit and subsequently, the first codon is translated. In eukaryotes, eukaryotic Initiation Factors 4E and 4G (eIF4E and eIF4G) bind to the 5′ end of the mRNA, recruiting both the smaller subunit of a ribosome and a tRNA carrying methionine. The ribosome scans the mRNA to locate the start codon, after which the initiation factors dissociate from the translation machinery.