10.4:
Initiation of Translation
On the mRNA, the start site for translation is crucial. If translation were to begin one nucleotide before or after the start codon, every codon that follows in the mRNA will be misread, synthesizing a non-functional sequence of amino acids.
Translation begins with the codon AUG, and an initiator tRNA that carries the amino acid Methionine or, the chemically modified formylmethionine in bacteria.
The initiator tRNA also contains conserved nucleotides that are recognized by proteins called eukaryotic initiation factors, or eIFs.
Together with eIF2 and GTP, the initiator tRNA binds the P site of the small ribosomal subunit forming the eukaryotic pre-initiation complex.
This complex recognizes the mRNA by interacting with initiation factors eIF4E bound to the 5’ cap, and eIF4G bound to the poly(A) tail-binding proteins.
Powered by ATP hydrolysis, the complex then moves from 5ʹ to 3ʹ direction, with the tRNA anticodon searching for the first AUG sequence on the mRNA.
Upon codon-anticodon recognition, GTP is hydrolyzed and the initiation factors dissociate, allowing the large ribosomal subunit to join the complex and form an intact ribosome.
Now, a new tRNA, carrying the second amino acid, can bind to the A-site on the ribosome and protein synthesis can begin.
In bacteria, mRNAs do not have 5’ caps to initiate translation.
Instead, each bacterial mRNA contains a leader sequence upstream of the first AUG codon, called the Shine-Dalgarno sequence.
This consensus AGGAGGU sequence serves as the ribosomal binding site by base pairing with a complementary sequence on the 16S rRNA of the small ribosomal subunit.
Now, the 50S ribosomal subunit can bind to the initiation complex, with the complete ribosome ready to begin translation.
10.4:
Initiation of Translation
Initiating translation is complex because it involves multiple molecules. Initiator tRNA, ribosomal subunits, and eukaryotic initiation factors (eIFs) are all required to assemble on the initiation codon of mRNA. This process consists of several steps that are mediated by different eIFs.
First, the initiator tRNA must be selected from the pool of elongator tRNAs by eukaryotic initiation factor 2 (eIF2). The initiator tRNA (Met-tRNAi) has conserved sequence elements including modified bases at specific positions. This makes Met-tRNAi different from other
tRNAs carrying Methionine.
Next, the eIF2/GTP/Met-tRNAi ternary complex and other eIFs bind to the small ribosomal subunit to form a 43S preinitiation complex. Before the preinitiation complex binds the mRNA, to make sure that a correctly processed mRNA is translated, the cell uses initial recognition of the 5’ cap of the mRNA by the eIF4E subunit of eIF4F.
Most eukaryotic mRNAs are monocistronic, that is, they encode only a single protein. Once the preinitiation complex is bound to the mRNA, the complex moves forward to search for the first AUG triplet, which is usually 50–100 nucleotides downstream of the 5′-terminal cap.
During this scan, the nucleotides adjacent to the start codon affect the efficiency of codon recognition. If the recognition site is substantially different from the consensus recognition sequence (5ʹ-ACCAUGG-3ʹ), the preinitiation complex may skip over the first AUG triplet in the mRNA and continue scanning to the next AUG. This phenomenon is known as “leaky scanning.” Several viruses, such as the human papillomavirus, use leaky scanning as the predominant mechanism during translation. Other cells also frequently use this to produce multiple proteins from the same mRNA molecule.
Bacterial ribosomes can assemble directly on a start codon that lies several nucleotides downstream of the Shine-Dalgarno sequence. So, a single molecule of bacterial mRNA can code for several different proteins, which makes bacterial mRNAs polycistronic.
Leitura Sugerida
- Alberts, Bruce. "Molecular Biology of the Cell." (2016), Pgs 347-348.
- Pestova, Tatyana V., Victoria G. Kolupaeva, Ivan B. Lomakin, Evgeny V. Pilipenko, Ivan N. Shatsky, Vadim I. Agol, and Christopher UT Hellen. "Molecular mechanisms of translation initiation in eukaryotes." Proceedings of the National Academy of Sciences 98, no. 13 (2001): 7029-7036.
- Stacey, Simon N., Deborah Jordan, Andrew JK Williamson, Michael Brown, Joanna H. Coote, and John R. Arrand. "Leaky scanning is the predominant mechanism for translation of human papillomavirus type 16 E7 oncoprotein from E6/E7 bicistronic mRNA." Journal of virology 74, no. 16 (2000): 7284-7297.