Protein synthesis in prokaryotes

Protein synthesis in prokaryotes is in principle the same as in eukaryotes. However, as the process is simpler and has been better studied in prokaryotes, the details involved in translation are discussed here and on p. 252 using the example of the bacterium Escherichia coli. 

The molecular and genetic relationship between enzyme induction and repression was clarified by the genetic research of Jacob and Monod at the Pasteur Institute, Paris (see reference l7>). Their classic work led them to develop the operon hypothesis for the control of protein synthesis in prokaryotes, which has since been verified by direct biochemical experiments. 

Formation of the initiation complex for protein synthesis in prokaryotes. E. coli has three initiation factors bound to a pool of 30S ribosomal subunits. One of these factors, IF-3, holds the 30S and 50S subunits apart after termination of a previous round of protein synthesis. The other two factors, IF-1 and IF-2, promote the binding of both fMet-tRNAfMel and mRNA to the 30S subunit. The binding of mRNA occurs so that its Shine-Dalgamo sequence pairs with 16S... 

Allen, T.E. and Palsson, B.O. (2003) Sequence-based analysis of metabolic demands for protein synthesis in prokaryotes. J. Theor. Biol. 220,1-18. 

Transfer RNA molecules (tRNAs), messenger RNA, and many proteins participate in protein synthesis along with ribosomes. The link between amino acids and nucleic acids is first made by enzymes called aminoacyl-tRNA synthetases. By specifically linking a particular amino acid to each tRNA, these enzymes implement the genetic code. This chapter focuses primarily on protein synthesis in prokaryotes because it illustrates many general principles and is relatively well understood. Some distinctive features of protein synthesis in eukaryotes also are presented. 

Regulation of protein synthesis in prokaryotes occurs mainly at the transcriptional level and involves genetic units known as operons. 

C. mRNA supplies the codons, aminoacyl-tRNA and GTP provide energy, peptidyl transferase catalyzes the formation of peptide bonds, and elongation factor 2 translocates the peptidyl-tRNA. Formylmethionyl-tRNA is involved in initiation of protein synthesis in prokaryotes. 

C. These antibiotics inhibit protein synthesis in prokaryotes thus, they can be used to treat bacterial infections. One of their undesirable side effects, however, is that they also inhibit protein synthesis on mitochondrial ribosomes (which are of the 70S prokaryotic class). 

C. Erythromycin prevents translocation during protein synthesis in prokaryotes. 

The mechanismsof protein synthesis in prokaryotes and eukaryotes differ slightly in detail, but the prokaryotic mechanism is used as a general model ... 

Early steps in protein synthesis in prokaryotes formation of the 30S preinitiation complex and 70S initiation complex. 

Synthesis of formylmethionyl-tRNA (N -formyl FH4) required for initiation of protein synthesis in prokaryotes (Chapter 25) and in mitochondria and... 

Many antibiotics act to inhibit protein synthesis in prokaryotes without affecting eukaryotic cells much. 8everal important targets in protein synthesis have been identified that are blocked by these agents to curb microorganism... 

The Regulation of Protein Synthesis in Prokaryotes (Figure 27.31, Figure 27.32, Figure 27.33)... 

Many antibiotics that are used to combat bacterial infections in humans take advantage of the differences between the mechanisms for protein synthesis in prokaryotes and eukaryotes. For example, streptomycin binds to the 30S ribosomal subunit of prokaryotes. It interferes with initiation of protein synthesis and causes misreading of mRNA. 

Protein synthesis in prokaryotes takes place as a coupled process with simultaneous transcription of mRNA and translation of the message in protein synthesis. This is possible because of the lack of compartmentaliza-tion in prokaryotic cells. In eukaryotes, mRNA is transcribed and processed in the nucleus and only then exported to the cytoplasm to direct protein synthesis. 

The first amino acid that starts the process of protein synthesis in prokaryotic (bacterial) cells is a derivative of methionine. This compound, A -formylmethionine, initiates the growing polypeptide chain as the N-tenninal amino acid. The fact that most proteins do not have Af-formylmethionine as the N-terminal amino acid indicates that when protein synthesis is completed, the iV-formylmethionine is cleaved from the finished protein. 

Fig. 2. Formation of a stable initiation complex between a 70 S ribosome and messenger RNA. In the final complex fMet-tRNAf " is in the correct position for the formation of a peptide bond. IF-1, IF-2, and IF-3 are the protein initiation factors and fMet-tRNAf " is the formyl methionyl tRNA which is used for the initiation of protein synthesis in prokaryotes. The process in animal cells is thought to be substantially the same, the initiation factors being termed IF-Ml, IF-M2, and IF-M3 and the initiator tRNA, Met-tRNAt . The methionine attached to this tRNA species is not normally formylated but can be so modified by enzymes from bacterial cells.

The temporal genome organization of eukaryotes could have evolved from the functional organization of the prokaryotic genome. The temporal sequence of protein synthesis in prokaryotes occurs from a stable, polycistronic template which is determined by the linear sequence of cistrons (Ohtaka and Spiegelman, 1963). For instance, the enzymes for histidine synthesis are in a sequence which corresponds to the linear gene sequence in the histidine operon of Salmonella typhimurium. There is a 20 min period between the appearance of the first and last (tenth) enzymes. The data can be explained on the basis of the successive synthesis of individual mRNA s or on the successive synthesis of a polycistronic message. This... 

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