Translation in prokaryotes

Gold, L., Posttranslational regulatory mechanisms in E. coli. Ann. Rev. Biochem. 57 199, 1988. A summary of the mechanisms of initiation and regulation of translation in prokaryotic systems. 

Translation in prokaryotes (H2) Protein glycosylation (H5) Translation in eukaryotes (H3)... 

The ribosome has a problem similar to that of RNA polymerase the ribosome must find the point in the mRNA at which to begin translation. In prokaryotes the site is marked by a tract called the Shine-Dalgarno sequence, about ten nucleotides upstream from the initiation site. Initiation occurs at the first subsequent AUG sequence. (AUG codes for the amino acid methionine.) In eukaryotes, initiation usually begins simply at the first AUG sequence from the 5 -end of the mRNA. 

Coupled transcription-translation in prokaryotes refers to the commencement of translation of an RNA molecule before its transcription from the DNA template is complete. Could such a situation arise in eukaryotes ... 

Translation in prokaryotes begins with the formation of the ribosome complex at a defined position on the mRNA, termed the Shine-Delgamo sequence, or the ribosome binding site (RBS). In prokaryotic mRNA, this is a relatively small (4-7 nucleotides) region rich in purine nucleotides located less than 10 nucleotides to the 5 side of the translational start site (Fig. 23-3). This Shine-Delgarno sequence is complementary to the 16S ribosomal RNA (rRNA) associated with the 30S ribosomal subunit, and directs it to bind the mRNA at that position. The 3 OS ribosomal subunit will not bind this region on the mRNA without the aid of an associated protein called initiation factor 3 (IF-3). The 30S ribosomal subunit will bind the mRNA in such a manner that the peptidyl (P) site of the complex is occupied by a specialized codon with the sequence, AUG. It is at this AUG start codon where translation will eventually begin. 

M. Kozak. 1999. Initiation of translation in prokaryotes and eukaryotes Gene 234 187-208. (PubMed)... 

Gene expression can also be regulated at the level of translation. In prokaryotes, many operons important in amino acid biosynthesis are regulated by attenuation, a process that depends on the formation of alternative structures in mRNA, one of which favors transcriptional termination. Attenuation is mediated by the translation of a leader region of mRNA. A ribosome stalled by the absence of an aminoacyl-tRNA needed to translate the leader mRNA alters the structure of mRNA so that RNA polymerase transcribes the operon beyond the attenuator site. 

Eukaryotes contain nuclei. Therefore, transcription is separated from translation. In prokaryotes, transcription and translation occur simultaneously. 

A FIGURE 4-12 Comparison of gene organization, transcription, and translation in prokaryotes and eukaryotes. 

A second level of control of the tryptophan biosynthetic pathway was discovered by Charles Yanofsky when he characterized mutants in the trp operon that did not affect Trp repressor binding. Yanofsky and his colleagues characterized a novel form of transcriptional control they called attenuation, which depends on the unique linkage between transcription and translation in prokaryotes. As shown in Figure 28.11, the intracellular concentration of TRP-tRNATrp determines if the ribosome will pause at a set of codons in the trp mRNA that specify consecutive Trp residues. When tryptophan levels are high, and TRP-tRNATrp is available, then the transcriptional termination hairpin loop forms and RNA polymerase disengages from the DNA template just downstream of a polyuridine... 

Protein translation is the process of synthesizing proteins from amino acids. This series of reactions translates the code provided to messenger ribonucleic acid or RNA (mRNA) by deoxyribonucleic acid or DNA into a sequence of amino acids that makes up the active protein molecule. Protein synthesis begins with a strand of mRNA synthesized in response to the genetic code located in a gene on a strand of DNA. The process of translation is slightly different in eukaryotic cells from that in prokaryotic cells for the sake of simplicity, translation in prokaryotes will be discussed here. 

Many of the differences between translation in prokaryotes and eukaryotes can be seen in the response to inhibitors of protein synthesis and to toxins. The antibiotic chloramphenicol (a trade name is Ghloromycetin) binds to the A site and inhibits peptidyl transferase activity in prokaryotes, but not in eukaryotes. This property has made chloramphenicol useful in treating bacterial infections. In eukaryotes, diphtheria toxin is a protein that interferes with protein synthesis by decreasing the activity of the eukaryotic elongation factor eEF2. 

Eukaryotic translation involves many more protein factors than the corresponding translation in prokaryotes. 

How does the ribosome know where to start translating In prokaryotic translation, the correct AUG start codon is identified by its proximity to a consensus sequence called the Shine-Dalgarno sequence. This sequence is complementary to a sequence on the small subunit of the prokaryotic ribosome. The ribosome is initially positioned on the Shine-Dalgarno sequence, which aligns it for correct translation initiation at the start codon. 

The critical role that protein factors play in translation is discussed next, including initiation, elongation, and release factors. The termination of translation is outlined, and the role of release factors that recognize translation stop codons is described. The chapter closes with a brief overview of translation in eukaryotes, emphasizing the major contrasting features with respect to translation in prokaryotes. Differences in the initiator tRNA, the selection mechanism of the initiator codon, the ribosomes, and the overall complexity of the process are highlighted. Last, the mechanisms of several potent inhibitors of translation and the mechanism of the bacterial toxin that causes diphtheria is presented. 

The phenomenon of attenuation of translation of the trp operon in bacteria is provided as an example of posttranscriptional gene regulation. This mechanism, which is used by several amino acid biosynthetic operons, relies on alternative RNA secondary structures and on the coupling of transcription and translation in prokaryotes. The regulation of iron metabolism in animals is presented to show how RNA secondary structures can by bound specifically by proteins and thereby regulate translation. 

Because we have a clearer idea of the way in which ribosomes interact with mRNA and assemble polypeptides in bacteria than in eukaryotes, here the bacterial system is considered first. The overall process in eukaryotes is very similar, and the special features of eukaryotes are covered later. Translation of an RNA message into a polypeptide occurs in three stages initiation, elongation, and termination. Translation in prokaryotes begins soon after transcription begins, and long before the 3 end of mRNA is completed, thus confirming that mRNA is read from 5 —>3. Initiation in bacteria involves the interaction of the 30S ribosomal subimit at the appropriate location on mRNA. 

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