tRNA molecules in the P (peptide) and A (acceptor) sites of the ribosome. During the process, the forming peptide is moved to the P site and the most 5' tRNA is released from the E (exit) site. This movement vacates the A site, which contains the codon specific for thé next amino acid, so that the incoming tRNA-amino acid can join the complex (see Figure 2-6, C).
Because multiple proteins encoded on an mRNA strand can be translated at the same time, multiple ribosomes may be simultaneously associated with one mRNA molecule. Such an arrangement is referred to as a polysome; its appearance resembles a string of pearls.
Tennination, the final step in translation, occurs when the ribosomal A site encounters a stop or nonsense codon that does not specify an amino acid (i.e., a "stop signal"; Table 2-1). At this point, the protein synthesis complex disassociates and the ribosomes are available for another round of translation. Following tennination, most proteins must undergo some extent of modification such as folding or enzymatic trimming so that protein function, transportation, or incorporation into various cellular structures can be accomplished. This process is referred to as posttranslational modification.
The vital role that gene expression and protein synthesis play in the survival of cells dictates that bacteria judiciously control these processes. The cell must regulate gene expression and control the activities of gene products so that a physiologic balance is maintained. Regulation and control are also key and highly complex mechanisms by which single-cell organisms are able to respond and adapt to environmental challenges, regardless of whether the challenges occur naturally or result from medical progress (e.g., antibiotics).
Regulation occurs at one of three levels of the gene expression and protein synthesis pathway: transcriptional, translational, and posttranslational. The most common is transcriptional level regulation. Because direct interactions with genes and their ability to be transcribed to mRNA are involved, transcriptional level regulation is also referred to as genetic level control. Genes that encode enzymes involved in biosynthesis (anabolic enzymes) and genes that encode enzymes for biodégradation (catabolic enzymes) will be used as examples of genetic level control.
In general, genes that encode anabolic enzymes for the synthesis of particular products are repressed
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