The process immediately preceding protein synthesis at the ribosome is peptidyl transferase. This process leads to addition of a water soluble amino acid onto the tRNA. This is the second step of this conversion and can be broken down into two processes: aminoacylation, and anti-codon-anticodon binding. During the first step, an amino acid is attracted by electrostatic interactions to its tRNA.
Translation of the mRNA transcript at the ribosome.
Protein translation at the ribosome begins with initiation, and ends with termination. The process of translation is divided into three phases; initiation, elongation and termination.
Initiation:
mRNA is bound to a small subunit of the ribosome. The small subunit then moves along the mRNA transcript until it encounters a start codon (AUG). At this point, the small subunit and mRNA are joined by the large subunit of the ribosome. These events are dependent on initiation factors that are associated with the mRNA transcript and ribosomal subunits.
Elongation:
During elongation, tRNA bring amino acids to the ribosome during translation in order to form a polypeptide chain based on the sequence of codons present in the mRNA transcript. Elongation involves three separate steps; binding of tRNA, formation of peptide bonds between adjacent amino acids, and translocation of tRNA along the mRNA transcript to allow for more amino acids to be added to an extending polypeptide chain.
Transcription and processing of the primary RNA transcript at the nucleus.
The ribosome is the largest complex of macromolecules in the cell and is responsible for protein synthesis in the cytoplasm. However, before any proteins can be synthesized by the ribosome, the primary RNA transcript must first be transcribed and processed in the nucleus.
In eukaryotes, DNA is packaged into chromosomes inside the nucleus, so the process of transcription must take place within this organelle. Transcription begins with an RNA polymerase protein that identifies a gene to express and uses one strand of DNA as a template from which to build an RNA molecule.
The specific sequence of RNA nucleotides that results from this process depends on the gene being expressed. The newly created polymeric RNA molecule is called a primary RNA transcript.
After being transcribed, the primary transcript is modified through a series of processing steps that are different depending on whether it will function as messenger RNA (mRNA), transfer RNA (tRNA), or ribosomal RNA (rRNA).
mRNA molecules are processed to remove non-coding sequences at both ends called introns and retain only coding sequences known as exons. These modifications are necessary to allow mRNA molecules to be translated into proteins by ribosomes in the cytoplasm.
Localization of mRNAs and ribosomes to a specific region within the cell.
The ribosome is the site where protein synthesis takes place. That is, it is the site where protein translation occurs. Before translation can occur, however, an mRNA molecule must be assembled. The mRNA is complementary to one of the DNA strands, and thus will contain the code for a single polypeptide chain.
Translation requires:
The mRNA molecule to be localized to a specific region within the cell (the ribosomes), so that translation can take place.
Activation of amino acids so that they can be incorporated into proteins. This is achieved via aminoacyl tRNAsynthetases. These enzymes recognize a codon on the mRNA and select a specific tRNA molecule with an anticodon that recognizes this codon and which carries the correct amino acid at its 3′ end.
Processing of rRNA in the nucleolus.
Processing of rRNA in the nucleolus. Ribosomal RNA (rRNA) synthesis is localized to the nucleolus, which is a subnuclear structure formed by phase separation.
The nucleolus is composed of distinct domains and subdomains, each of which contains proteins that work together to form the ribosome. In the nucleolus, rRNA associates with ribosomal proteins and forms a complex called a pre-ribosome. The association of RNA and protein takes place in several steps catalyzed by trans-acting factors.
These factors are required for proper folding and assembly of rRNAs into pre-ribosomes. The assembly process also involves modification of rRNA by methylation and pseudouridylation, which are catalyzed by small nucleolar RNAs (snoRNAs) and their associated proteins.
Upon completion of assembly, pre-ribosomes are transported out from the nucleolus to the cytoplasmic side of the nuclear envelope where they become mature functional ribosomes.
Activation of aminoacyl tRNAsynthetases by EF-Tu during elongation.
Aminoacyl-tRNAsynthetases (aaRSs) catalyze the activation of amino acids by ATP and their coupling to tRNAs. The EF-Tu dependent aaRS activation mechanism is conserved in all organisms, but the details of this process vary depending on whether the organism is prokaryotic or eukaryotic.
Both prokaryotic and eukaryotic aaRSs have similar structural domains; a catalytic domain at the C-terminus that contains the active site, and an acceptor stem domain at the N-terminus that binds tRNA. Additionally, both types of aaRSs are regulated by GTP binding proteins during elongation, however, while bacteria use EF-Tu, eukaryotes use the homologous protein EF-1α.
The presence of two GTP binding proteins in bacterial translation has allowed for the development of two alternative mechanisms for aminoacyl tRNA formation from uncharged tRNA and an amino acid: direct charging and EF-Tu dependent charging.
Direct charging occurs when EF-Tu activity is inhibited by antibiotic drugs such as puromycin or paromomycin, which causes charged tRNAs to be loaded onto ribosomes without
Last Words
You must be wondering what process immediately precedes protein synthesis and how RNA helps this happen. Ribosomal RNA is integral to protein synthesis, and responsible for recognizing codons in messenger RNA, as well as attaching amino acids together with peptide bonds. It’s a pretty simple system, but one that researchers are still trying to understand.