Termination

Stop codon

Release factor

Release factor binds to the complex when a stop codon enters the A site.

Releasing the polypeptide product: The release factor frees the tRNA from the P site and disconnects the polypeptide.

'C terminus / N terminus

Release factor binds to the complex when a stop codon enters the A site.

Releasing the polypeptide product: The release factor frees the tRNA from the P site and disconnects the polypeptide.

'C terminus / N terminus

Small subunit

Small subunit

Hl The remaining components (mRNA and ribosomal subunits) separate.

Large subunit

12.12 The Termination of Translation Translation terminates when the A site of the ribosome encounters a stop codon on the mRNA.

Hl The remaining components (mRNA and ribosomal subunits) separate.

Large subunit

12.12 The Termination of Translation Translation terminates when the A site of the ribosome encounters a stop codon on the mRNA.

they bind tRNAs. Rather, they bind a protein release factor, which hydrolyzes the bond between the polypeptide and the tRNA in the Psite.

The newly completed protein thereupon separates from the ribosome. Its C terminus is the last amino acid to join the chain. Its N terminus, at least initially, is methionine, as a consequence of the AUG start codon. In its amino acid sequence, it contains information specifying its conformation, as well as its ultimate cellular destination.

Table 12.1 summarizes the nucleic acid signals for initiation and termination of transcription and translation.

Regulation of Translation

Like any factory, the machinery of translation can work at varying rates. Variation in the rate of translation is useful for controlling the amount of an active protein in a cell. Some externally applied chemicals, such as some antibiotics, can stop translation. Conversely, the presence of more than one ribo-some on an mRNA can speed up protein synthesis.

Some antibiotics and bacterial toxins work by inhibiting translation

Antibiotics are defensive molecules produced by microorganisms such as certain bacteria and fungi. These substances often destroy other microbes, which might compete with the defenders for nutrients. Since the 1940s, scientists have isolated increasing numbers of antibiotics, and physicians use them to treat a great variety of infectious diseases, ranging from bacterial meningitis to pneumonia to gonorrhea.

The key to the medical use of antibiotics is specificity: An antibiotic must act to destroy the microbial invader, but not harm the human host. One way in which antibacterial antibiotics achieve this is to block the synthesis of the bacterial cell wall— something that is essential to the microbe but is not part of human biochemistry. Penicillin works in this way.

Another way in which antibiotics work is to inhibit all bacterial protein synthesis. Recall that the prokaryotic ribosome is smaller, and has a different collection of proteins, than the eukaryotic ribosome. Some antibiotics bind only to bacterial ribosomal proteins that are important in protein synthesis (Table 12.2). Without the ability to make proteins, the bacterial invaders die, and the infection is stemmed.

Some bacteria affect their human hosts through mechanisms similar to those we use against them. Diphtheria is an infectious disease of childhood, and before the advent of effective vaccines, it was a major cause of childhood death. The infective agent, the bacterium Cornybacterium diphtheriae, produces a highly lethal toxin that modifies and inactivates

Signals that Start and Stop Transcription 12» 1 and Translation

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