RNA translation in bacteria

Because we have a clearer picture of the way in which ribosomes interact with mRNA and assemble polypeptides in bacteria, this will be considered in some detail first. The overall process in eukaryotes is very similar, and the special features of eukaryotes will be treated in the section that follows. 

Translation of an RNA message into a polypeptide occurs in three stages initiation, elongation, and termination. As already mentioned, initiation in bacteria involves the interaction of the 30S ribosomal subunit at the appropriate location on mRNA. 

Question What features of mRNA structure enable interaction with the 30S subunit  

Toward the 5 end of mRNA, there is a region of 20 or so nucleotides before the initiation codon AUG is reached. This leader region contains a sequence responsible for interaction of the mRNA with the 30S subunit. It is known as the Shine-Dalgarno (S-D) sequence, and it can bind to a complementary sequence at the 3 end of the 16S rRNA to position the 30S subunit appropriately for initiation. Other sequences in the leader region are possibly involved in the overall process of initiation of translation, which involves also the binding of the appropriately charged methionyi-tRNA opposite the AUG codon. 

Question Is there any special feature of methionyl-tRNA, in addition to the presence of an anticodon for AUG, that is required for its participation in initiation of translation  

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Knowled of individual enzymes and receptor-ligand interactions now provides the opportunity for a more directed approach to drug discovery and development. The rational mode of anti-microbial drug discovery has started to bear fhiit with the identification of small molecule inhibitors of the anthrax lethal factor, and the structure-guided discovery of a new inhibitor of RNA translation in bacteria. ... 

FIGURE 27-28 Coupling of transcription and translation in bacteria. The mRNA is translated by ribosomes while it is still being transcribed from DNA by RNA polymerase. This is possible because the mRNA in bacteria does not have to be transported from a nucleus to the cytoplasm before encountering ribosomes. In this schematic diagram the ribosomes are depicted as smaller than the RNA polymerase. In reality the ribosomes (Mr 2.7 X 105) are an order of magnitude larger than the RNA polymerase (Mr 3.9 X 105). 

Thomas That is not absolutely true. In bacteria, if you change the energy charge you see a direct effect on translation and ribosome biosynthesis. As soon as ATP drops, ribosomal RNA synthesis decreases but not transcription of other genes. 

Cold-shock proteins (Csps) are transiently expressed at a higher level in bacteria as a response to an abrupt decrease from normal physiological temperature. Their exact biological function is still unknown, but translational regulation, possibly via RNA chaperoning, has been discussed. " Csps from mesophiles and thermophiles differ widely in their stability, but show... 

In bacteria, transcription and translation are tightly coupled. Messenger RNAs are synthesized and translated in the same 5 — 3 direction. Ribosomes begin translating the 5 end of the mRNA before transcription is complete (Fig. 27-28). The situation is quite different in eukaryotic cells, where newly transcribed mRNAs must leave the nucleus before they can be translated. 

In bacteria transcription and translation are closely linked. Polyribosomes may assemble on single DNA strands as shown in Fig. 28-5. It has often been assumed that RNA synthesis occurs on loops of DNA that extend out into the cytosol. However, recent studies indicate that most transcription occurs in the dense nucleoid and that assembly of ribosomes takes place in the cytosol.2683 In a similar way eukaryotic transcription occurs in the nucleus and protein synthesis in the cytosol. Nevertheless, some active ribosomes are present in the nucleus.26813... 

One of the most prominent features of the 50S subunit is the LI protuberance, seen on the left side in Fig. 29-6A. This protuberance is formed almost entirely by protein LI, which is one of the largest ribosomal proteins. It binds to the 2105-2184 loop in domain V of the 23S RNA (see Fig. 29-14).139 LI has an important regulatory role in bacteria in which it represses translation of its own structural gene by binding to a region in its mRNA close to the Shine-Dalgamo sequence. 

The phosphate backbone of both DNA and RNA provides a negatively charged template for attracting positively charged species. Drugs that commonly exploit this interaction are the aminoglycoside antibiotics. Examples include neomycin B (6.20) and kanamycin A (6.21) (Figure 6.12). This class of compounds binds rRNA in bacteria to interfere with translation of mRNA into functional proteins. 

The function of the leader sequence is to fine tune expression of the trp operon based on the availability of tryptophan inside the cell. It does this as follows. The leader sequence contains four regions (Fig. 2, numbered 1-4) that can form a variety of base-paired stem-loop ( hairpin ) secondary structures. Now consider the two extreme situations the presence or absence of tryptophan. Attenuation depends on the fact that, in bacteria, ribosomes attach to mRNA as it is being synthesized and so translation starts even before transcription of the whole mRNA is complete. When tryptophan is abundant (Fig. 2a), ribosomes bind to the trp polycistronic mRNA that is being transcribed and begin to translate the leader sequence. Now, the two trp codons for the leader peptide lie within sequence 1, and the translational Stop codon (see Topic HI) lies between sequence 1 and 2. During translation, the ribosomes follow very closely behind the RNA polymerase and synthesize the leader peptide, with translation stopping eventually between sequences 1 and 2. At this point, the position of the ribosome prevents sequence 2 from interacting with sequence 3. Instead sequence 3 base-pairs with sequence 4 to form a 3 4 stem loop which acts as a transcription terminator. Therefore, when tryptophan is present, further transcription of the trp operon is prevented. If, however, tryptophan is in short supply (Fig. 2b), the ribosome will pause at the two trp codons contained within sequence 1. This leaves sequence 2 free to base pair with sequence 3 to form a 2 3 structure (also called the anti-terminator),... 

Most of the DNA sequences which are transcribed give rise to mRNA, which is subsequently translated into protein. However, the most abundant species of RNA are ribosomal RNA (rRNA) and transfer RNA (tRNA), which do not code for protein but function in the process of translation. They are formed by a high level of transcription of a relatively small number of genes (called rRNA and tRNA genes). In bacteria, transcription of all genes is brought about by the enzyme RNA polymerase. 

The genetic code is universal. All biological systems (or nearly all) use the same code. This principle has important consequences to the developments in rDNA. Thus, the code in messenger RNAs of a human cell can be translated by bacterial protein synthesis machinery into a protein of the same sequence as in the human cell. Production of human proteins in bacteria and other organisms would be immensely difficult, if not impossible, if each organism had a different genetic code. 

Deoxyribonucleic acid (DNA), the genetic material of bacteria and eukaryotic cells, is either copied into new complementary DNA or transcribed into ribonucleic acid (RNA), also of complementary sequence. It is then used as a template in the so-called translation into proteins. Some viruses use RNA as genetic material that is copied into new complementary RNA, or in leukemie viruses transcribed into DNA the latter process is called reverse transcription. 

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