Ribosome subunits sedimentation coefficients

The ribosomal subunits migrate through the nuclear pores into the cytoplasm where they complex with mRNA, forming 80S ribosomes. Because sedimentation coefficients reflect both shape and particle weight, they are not additive. 

Ribosomes are ancient ribonucleoprotein complexes that are the sites of protein synthesis in living cells. Their core structures and fundamental functional mechanisms have been conserved throughout the three domains of life bacteria, archaea and eukaryotes. All ribosomes are organized into two subunits that are defined by their apparent sedimentation coefficient, measured in Svedberg units (S). There is a general... 

Sedimentation coefficients are expressed in Svedbergs (S), after the Swedish biochemist The Svedberg who developed the ultracentrifuge in the 1920s. While S values are indicative of molecular weight, they are not addi-tive-the 70s ribosome is made up of one 50S and one 30S subunit. 

Figure 1.7 Structure of a ribosome. It is composed of two subunits the large 60S subunit has a mass of 2800 kDa and is composed of three RNA molecules and about 50 protein molecules. The smaller AOS subunit contains one RNA molecule plus around 30 protein molecules and has an aggregate mass of 1400 kDa. ( S is an abbreviabon for a Svedberg, the unit of the sedimentation coefficient. This is measured in an analybcal ultracentrifuge and is related to, but not simply proporbonal to, molecular mass.)...

Ribosomes consist of two subunits of different size, made up of ribosomal RNA (rRNA) and neariy 80 proteins (the number of proteins appiies to rat iiver ribosomes), it is customary to give the sedimentation coef cients (see p. 200) of ribosomes and their components instead of their masses. For exampie, the eukaryotic ribosome has a sedimentation coefficient of 80 Svedberg units (80 S), whiie the sedimentation coef cients of its subunits are 40 S and 60 S (S vaiues are not additive). 

Each E. coli cell contains 15,000 or more ribosomes, making up almost a quarter of the dry weight of the cell. Bacterial ribosomes contain about 65% rRNA and 35% protein they have a diameter of about 18 nm and are composed of two unequal subunits with sedimentation coefficients of 30S and 50S and a combined sedimentation coefficient of 70S. Both subunits contain dozens of ribosomal proteins and at least one large rRNA (Table 27-6). 

The ribosomes of eukaryotic cells (other than mitochondrial and chloroplast ribosomes) are larger and more complex than bacterial ribosomes (Fig. 27-9d), with a diameter of about 23 nm and a sedimentation coefficient of about 80S. They also have two subunits, which vary in size among species but on average are 60S... 

Ribosomes are large complexes of protein and rRNA (Figure 31.8). They consist of two subunits—one large and one small—whose relative sizes are generally given in terms of their sedimentation coefficients, or S (Svedberg) values. [Note Because the S values are determined both by shape as well as molecular mass, their numeric values are not strictly additive. For example, the prokaryotic 50S and 30S ribosomal subunits together form a ribosome with an S value of 70. The eukaryotic 60S and 40S subunits form an 80S ribosome.] Prokaryotic and eukaryotic ribosomes are similar in structure, and serve the same function, namely, as the "factories" in which the synthesis of proteins occurs. 

The ribosomes of Escherichia coli (M = 2.3 x 106 dalton) consist of two dissimilar subunits. The larger one is the 50 S unit containing 32 unique proteins (L1-L34) and one molecule each of 5 S and 23 S rRNA. The smaller subunit has a sedimentation coefficient of 30 S and contains one molecule 16 S rRNA and 21 unique proteins (S1-S21). The proteins of both subunits could be separated by RPC on a C 18 column (250 x 4.6 mm do = 30 nm dP = 10 pm) by a gradient water/2-propanol with 0.1 % trifluoroacetic acid in both solvents (Fig. 20). With 2-propanol, 15 proteins of the... 

Whereas a prokaryotic ribosome has a sedimentation coefficient (see Topic G9) of 70S and subunits of 30S and 50S, a eukaryotic ribosome has a sedimentation coefficient of 80S with subunits of 40S and 60S (see Topic G9). The composition of eukaryotic ribosomal subunits is also more complex than prokaryotic subunits (see Topic G9) but the function of each subunit is essentially the same as in prokaryotes. 

Ribosomes are large macromolecular complexes whose components contain all the information necessary for self-assembly. The E. coli ribosome has a sedimentation coefficient of 70 S and consists of two subunits (50 S and 30 S) with a total mass of 2.8 x 106 Da and with 58 different components. Three of these components are RNA molecules that together comprise 65 percent of the mass and they act as a framework or template for the ordering of the different proteins. When the pure dissociated components are mixed together in the proper order under the correct conditions they spontaneously reassemble to form a fully active ribosome (Fig. 5-1). 

Ribosome s are found in the cytoplasm, on the outer face of the rough ER and in the mitochondrial matrix. Ribosome s are composed of RNA and proteins. The sedimentation coefficient of eukaryotic ribosomes is SOS. The sedimentation coefficient, S, is a unit of measure that describes how fast a macromolecule will sediment when spun in a high speed centrifuge. Larger molecules generally have larger S values. Eukaryotic ribosomes consist of two subunits a 60S large subunit and a 40S small subunit. The 60S subunit is comprised of about 45 proteins and three rRNA s that have S coefficients of 5, 5.8 and 28. The 40S subunit consists of about 33 proteins and 18S rRNA s. 

A typical eukaryotic ribosome has a sedimentation coefficient of about 80 and consists of two subunits 408 and 608. These sizes may vary by as much as 10% from one organism to another in contrast with the homogeneity of bacteria ribosomes. The components of the subunits of eukaryotic ribosomes are as follows ... 

The answer is c. (Murray, pp 452—467. Scriver, pp 3—45. Sack, pp 1—40. Wilson, pp 101-120.) Prokaryotic ribosomes have a sedimentation coefficient of 70S and are composed of SOS and 30S subunits. Eukaryotic cytoplasmic ribosomes, either free or bound to the endoplasmic reticulum, are larger—60S and 40S subunits that associate to an SOS ribosome. Nuclear ribosomes are attached to the endoplasmic reticulum of the nuclear membrane. Ribosomes in chloroplasts and mitochondria of eukaryotic cells are more similar to prokaryotic ribosomes than to eukaryotic cytosolic ribosomes. Like bacterial ribosomes, chloroplast and mitochondrial ribosomes use a formylated tRNA. In addition, they are sensitive to many of the inhibitors of protein synthesis in bacteria. 

Bacterial ribosomes have two subunits, known as 30s and 50s according to their sedimentation coefficient (which is a measure of how different fractions sediment during centrifugation). 

The ribosome is the structure on which translation occurs. Figure 27.13 shows that the prokaryotic ribosome consists of two large subunits. Each subunit, in turn, consists of multiple proteins and RNA. The VOS, 508, and 308 designations refer to centrifugation sedimentation coefficients, which reflect the size of each entity, but are not strictly additive. 

Ribosomes are subcellular ribonucleoprotein complexes on which protein synthesis occurs. Different types of ribosomes are found in prokaryotes and in the cytoplasm and mitochondria of eukaryotic cells (Fig. 12.20). Prokaryotic ribosomes contain three types of rRNA molecules with sedimentation coefficients of 16, 23, and 5S. The 30S ribosomal subunit contains the 16S rRNA complexed with proteins, and... 

Ribosomes and ribosomal RNA have been studied extensively via sedimentation coefficients. Most research on prokaryotic systems has been done with the bacterium Escherichia coli, which we shall use as an example here. An E. coli ribosome typically has a sedimentation coefficient of 70S. When an intact 70S bacterial ribosome dissociates, it produces a light SOS subunit and a heavy 50S subunit. Note that the values of sedimentation coefficients are not additive, showing the dependence of the S value on the shape of the particle. The 30S subunit contains a 16S rRNA and 21 different proteins. The 50S suhunit contains a 5S rRNA, a 23S rRNA, and 34 different proteins (Figure 9.26). For comparison, eukaryotic ribosomes have a sedimentation coefficient of SOS, and the small and large subunits are 40S and 60S, respectively. The small suhunit of eukaryotes contains an 18S rRNA, and the large suhunit contains three types of rRNA molecules 5S, 5.8S, and 28S. 

The processing of rRNAs is primarily a matter of methylation and of trimming to the proper size. In prokaryotes, there are three rRNAs in an intact ribosome, which has a sedimentation coefficient of 70S. (Sedimentation coefficients and some aspects of ribosomal structure are reviewed in the discussion of ribosomal RNA in Section 9.5.) In the smaller subunit, which has a sedimentation coefficient of SOS, one RNA molecule has a sedimentation coefficient of 16S. The BOS subunit contains two kinds of RNA, with sedimentation coefficients of 5S and 23S. The ribosomes of eukaryotes have a sedimentation coefficient of SOS, with 40S and 60S subunits. The 40S subunit contains an 18S RNA, and the 60S subunit contains a 5S RNA, a 5.8S RNA, and a 28S RNA. Base modifications in both prokaryotic and eukaryotic rRNA are accomplished primarily by methylation. 

In sedimentation velocity experiments, the centrifnge is rnn at high speeds (up to -80,000 rpm), and the rate of sedimentation of the bonndary between the solvent and the sedimenting protein is measured to yield the sedimentation coefficient of the molecule (or molecules). This value is related to the size and shape of the molecule and is measnred in Svedberg units (denoted by S, and formally it is equal to 10 s). E.g., the 30S subunit of the ribosome has a sedimentation coefficient of 30 S. The Svedberg of Sweden invented the analytical ultacentrifnge, and he received the 1926 Nobel Prize in Chemistry for his work characterizing the hydrodynamic and mass properties of proteins. 

It should be noted that the results reported above have been obtained for the specified ionic environment. It is known that this environment can affect the ribosome structure for example, removal of magnesium bound to E.coli ribosomes causes a conformational change (unfolding) of the two subunits with a modification of their sedimentation coefficients and radii of gyration " ). 

Protein biosynthesis takes place on ribosomes, which are small organelles present in all organisms. In prokaryotes (bacteria and blue-green algae) the ribosomes are somewhat smaller (sedimentation coefficient 70S) than those in the cytoplasm of eukaryotic cells (SOS), whereas ribosomes from chloroplasts and mitochondria have approximately the same size as prokaryotic ribosomes. All ribosomes consist of two subunits of unequal size those from SOS ribosomes sediment at 40S and 60S and those from 70S at 30S and SOS, respectively. Protein biosynthesis can occur only when the two subunits are associated and combined with messenger RNA. 

Ribosomal RNAs of the same sedimentation coefficient from various sources demonstrate a high degree of evolutionary conservation of structure. It is believed that rRNA features prominently in the assembly and function of ribosomal subunits. The... 

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