Translation is the third stage of protein synthesis and proceeds in four phases: activation, initiation, elongation and termination. Activation occurs when an amino acid is covalently bonded to a tRNA. Initiation follows when the ribosome binds to the 5' end of mRNA. This allows translation to build a particular protein. This process is halted once a stop codon (UAA, UAG, or UGA) is reached because no tRNA can bind to these codons. What does recognize this codon is a "release factor" protein that disassembles the ribosome/mRNA complex.
An illustration of this process is shown as follows:
Shown above is a ribosome translating a protein that is secreted into the endoplasmic reticulum. tRNAs are colored dark blue.
Bacterial Ribosome Binding Sites
A ribosome binding site (RBS) is a region 6-7 nucleotides upstream of the start codon AUG in prokaryotes called the Shine-Dalgarno sequence (5′–GGAGGU–3′). The ribosome will base pair with this site through its own rRNA as well at the start codon using tRNA. What makes this an interesting topic of research is the fact that the Shine-Dalgarno sequence is not the "optimal" RBS for all expression processes. In other words, a RBS affects the rate at which a particular Open Reading Frame (ORF) is translated. There are two general ways in which this happens: i) the rate at which ribosomes are recruited to the mRNA and initiate translation is dependent on the sequence of the RBS and ii) the RBS can also affect the stability of the mRNA, thereby affecting the number of proteins made over the lifetime of the mRNA.
RBS Sequence Design
Stability of mRNA
- Laursen BS, Sørensen HP, Mortensen KK, and Sperling-Petersen HU. . pmid:15755955.
Targeting DNA double-strand breaks with TAL effector nucleases.