Ribosome mRNA interaction with

Translation, the second step in getting from a gene to a protein, takes place in the cytoplasm. The mRNA interacts with a specialized complex called a ribosome, which "reads" the sequence of mRNA bases. Each sequence of three bases, called a codon, usually codes for... 

The direction of translation has important con.sequences. Recall that transcription also is in the 5. V direction (p. 827). If the direction of translation were opposite that of transcription, only fully synthesized niRNA could be translated. In contrast, because the directions are the same, mRNA can be translated while it is being synthesized. In prokaryotes, almost no time is lost between transcription and translation. The 5 end of mRNA interacts with ribosomes very soon after it is made, much before the... 

In the production of cytoplasmic ribosomes in human cells, one portion of the 45S rRNA precursor becomes the 18S rRNA that, complexed with proteins, forms the small 40S ribosomal subunit (Fig. 14.15, circle 4). Another segment of the precursor folds back on itself and is cleaved, forming 28S rRNA, hydrogen-bonded to the 5.8S rRNA. The 5S rRNA, transcribed from nonnucleolar genes, and a number of proteins complex with the 28S and 5.8S rRNAs to form the 60S ribosomal subunit (Fig. 14.15, circle 5). The ribosomal subunits migrate through the nuclear pores. In the cytoplasm, the 40S and 60S ribosomal subunits interact with mRNA, forming the 80S ribosomes on which protein synthesis occurs. 

The second major step of gene expression involves the translation of a protein, e.g., enzymes, structural proteins, secreted proteins, gene expression factors, etc., in which the mRNA interacts with ribosomes and amino acid activated transfer RNA to direet the synthesis of the protein coded for by the mRNA sequence. 

Messenger RNA (mRNA) (Section 28.11) A polynucleotide of ribose that reads the sequence of bases in DNA and interacts with tRNAs in the ribosomes to promote protein biosynthesis. 

A ribosome is a cytoplasmic nucleoprotein stmcture that acts as the machinery for the synthesis of proteins from the mRNA templates. On the ribosomes, the mRNA and tRNA molecules interact to translate into a specific protein molecule information transcribed from the gene. In active protein synthesis, many ribosomes are associated with an mRNA molecule in an assembly called the polysome. 

Biochemical and genetic experiments in yeast have revealed that the b poly(A) tail and its binding protein, Pablp, are required for efficient initiation of protein synthesis. Further studies showed that the poly(A) tail stimulates recruitment of the 40S ribosomal subunit to the mRNA through a complex set of interactions. Pablp, bound to the poly(A) tail, interacts with eIF-4G, which in turn binds to eIF-4E that is bound to the cap structure. It is possible that a circular structure is formed and that this helps direct the 40S ribosomal subunit to the b end of the mRNA. This helps explain how the cap and poly(A) tail structures have a synergistic effect on protein synthesis. It appears that a similar mechanism is at work in mammalian cells. 

The information contained in the DNA (i.e., the order of the nucleotides) is first transcribed into RNA. The messenger RNA thus formed interacts with the amino-acid-charged tRNA molecules at specific cell organelles, the ribosomes. The loading of the tRNA with the necessary amino acids is carried out with the help of aminoacyl-tRNA synthetases (see Sect. 5.3.2). Each separate amino acid has its own tRNA species, i.e., there must be at least 20 different tRNA molecules in the cells. The tRNAs contain a nucleotide triplet (the anticodon), which interacts with the codon of the mRNA in a Watson-Crick manner. It is clear from the genetic code that the different amino acids have different numbers of codons thus, serine, leucine and arginine each have 6 codewords, while methionine and tryptophan are defined by only one single nucleotide triplet. 

Figure 7.5 Model of ferritin (and erythroid a-aminolaevulinate synthase) translation/ribosome binding regulation by IRP. In (a), with IRP not bound to the IRE (1) binding of the 43S preinitiation complex (consisting of the small ribosomal 40S subunit, GTP and Met-tRNAMet) to the mRNA is assisted by initiation factors associated with this complex, as well as additional eukaryotic initiation factors (elFs) that interact with the mRNA to facilitate 43S association. Subsequently (2), the 43S preinitiation complex moves along the 5 -UTR towards the AUG initiator codon, (3) GTP is hydrolysed, initiation factors are released and assembly of the 80S ribosome occurs. Protein synthesis from the open reading frame (ORF) can now proceed. In (b) With IRP bound to the IRE, access of the 43S preinitiation complex to the mRNA is sterically blocked. From Gray and Hentze, 1994, by permission of Oxford University Press.

Intracellular replication of viral particles depends entirely upon successful intracellular transcription of viral genes with subsequent translation of the viral mRNA. Translation of viral or cellular mRNA is dependent upon ribosome formation. Normally, several constituent molecules interact with each other on the mRNA transcript, forming the smaller ribosomal subunit. Subsequent for-mation/attachment of the larger subunit facilitates protein synthesis. 

Once the amino acid has been bound to its tRNA, it can pass to the next phase of protein synthesis, involving its interaction with mRNA, which takes place on the ribosome, a molecular machine of enormous complexity. The ribosome of E. coli is a ribonucleoprotein assembly of molecular weight 2700 kDa, and sedimentation constant of 70S9. It is made up of roughly two-thirds RNA and one-third protein, and can be separated into a small (30S) and a large (50S) subunit. The 30S subunit contains 21 proteins and one 16S RNA molecule, while the large subunit has 34 different proteins and two RNA molecules, one 23S and one 5S. Despite its size and complexity, the structure of both ribosomal subunits has been determined to atomic resolution (Figure 4.32), and very recently the atomic structure of the 70S ribosome has been determined at 2.8 A resolution (Selmer et al., 2006). 

Some proteins bind directly to mRNA and act as translational repressors, many of them binding at specific sites in the 3 untranslated region (3 UTR). So positioned, these proteins interact with other translation initiation factors bound to the mRNA or with the 40S ribosomal subunit to prevent translation initiation (Fig. 28-32 compare this with Fig. 27-22). 

Interact with small ribosomal subunits, blocking access of the aminoacyl-tRNA to the mRNA-ribosome complex. 

Most of the chemical activity of ribosomes occurs in the interface between the 30S and 50S subunits. Entrance and exit tunnels for both mRNA and the amino-acylated tRNAs are formed between these subunits. The mRNA apparently moves across the platform as the tRNAs move from A to P to E sites experiencing codon selection (decoding) and peptidyltransferase activity. Many loop ends from 16S RNA interact with those of 23S RNA.41 88... 

Initiation (Figs. 29-10 and 29-11), elongation (Fig. 29-12), and termination are three distinct steps in the synthesis of a protein. A variety of specialized proteins are required for each stage of synthesis. Their sequential interaction with ribosomes can be viewed as a means of ensuring an orderly sequence of steps in the synthesis cycle. The rate of protein formation will depend upon the concentrations of amino acids, tRNAs, protein factors, numbers of ribosomes, and kinetic constants. The formation of specific proteins can also be inhibited by translational repressors, proteins that compete with ribosomes for binding to target mRNAs.287... 

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