Termination

(available 4/6/00 - Source website??)

The work of the ribosome ends when it reaches one of the three stop codons that are used: UAG, UAA or UGA. The stop codons were originally identified by mutations in bacteriophage T4. The first one identified was UAG and was called the "amber codon".

Here's a bit of history of the amber codon. In Benzer's lab at Caltech the search was on for a mutation that would allow a certain kind of phage mutant to grow. Seymour said that whoever identified the mutation, he would name it after him (in some versions of the story, it would be named after the discoverer's mother). The graduate student who isolated the mutation was a young man named Harris Bernstein. The name "Bernstein" in German means "amber". And so the UAG codon, known as a nonsense codon (later known as a stop codon), was named the amber codon. Later, the other two stop codons were called "ochre" (UAA) and "opal" (UGA) (sometimes called, "umber") to maintain the color metaphor. Harris Bernstein later became a famous molecular biologist.

The event of termination is essentially the same in both prokaryotes and eukaryotes. The only difference is the number of factors required. This figure shows a schematic of termination (this figure is not in the book, although the information is):

When the a site of the ribosome finds itself over one of the stop codons (in this case, the amber codon), there will be no corresponding aminoacyl-tRNA. Instead, there are release factors (RFs) that enter the A site. In prokaryotes there are three release factors (RF-1, RF-2, and RF-3), while in eukaryotes there is only one (eRF).

Here's is the lactose mRNA again, to show where all of the stop codons are in each of the protein reading frames:

Termination results in the release of the ribosome from the mRNA and the removal of the peptide from its link to the tRNA.