Polysomes biosynthesis

From the above discussion, it appears that zinc may have its primary effect on zinc-dependent enzymes that regulate the biosynthesis and catabolic rate of RNA and DNA. In addition, zinc may also play a role in the maintenance of polynucleotide conformation. Sandstead et al. (99) observed abnormal polysome profiles in the liver of zinc-deficient rats and mice. Acute administration of zinc appeared to stimulate polysome formation both in vivo and in vitro. This finding is supported by the data of Femandez-Madrid, Prasad, and Oberleas (42), who noted a decrease in the polyribosome content of zinc-deficient connective tissue from rats and a concomitant increase in inactive monosomes. 

The fungal cell contains a vast number of ribosomes, which are usually present in the form of polysomes — lines of ribosomes strung together by a strand of mRNA. Ribosomes are the site of protein biosynthesis. The system that mediates the export of proteins from the cell involves a number of membranous compartments including the Golgi apparatus, the endoplasmic reticulum and the plasmalemma. In addition, the vacuole is employed as a storage space where nutrients, hydrolytic enzymes... 

Messenger RNA combines with ribosomes to form polysomes containing several ribosome units, usually 5 (e.g., during hemoglobin synthesis), complexed to the messenger RNA molecule. This aggregate structure is the active template for protein biosynthesis. 

Figure 30-1. Schematic representation of protein biosynthesis. mRNA molecules join the ribosomes, R, to form polysomes. tRNA molecules react specifically with the corresponding of-amino acids to form amino acyl RNA molecules. The amino acyl RNAs are bound to polysomes, whereby the amino acids are joined on to peptide chains and the tRNAs and polysomes are again released.

Noll H (1969) Polysomes Analysis of structure and function. In Campbell PN, Sargent JR (eds) Techniques in protein biosynthesis, vol II. Academic Press, London New York, pp 101-179... 

Polyribosomes, polysomes the structural unit of Protein biosynthesis (see), consisting of several to many Ribosomes (see) attached along the length of a strand of mRNA. P. occur free or attached to the endoplasmic reticulum. The individual ribosomes are separated by 60-90 nucleotide units of the mRNA, i.e. the distance between them is 20-30 nm. Each ribosome covers about 35 nucleotide residues. Length of the P. is approximately proportional to the size of the synthesized polypeptide. P are formed by association of mRNA with ribosomal subunits (produced at the termination of translation) under the influence of initiation factor IF 3, or by direct association of mRNA with newly formed ribosomes. 

Ribosomes, mmuaomes the sites of Protein biosynthesis (see) in the cell. R. resemble giant multienzyme complexes. They are spherical to ellipsoid, highly hydrated cell organelles, 15 to 30 run in diameter, normally present in the cytoplasm as Polysomes (see). They were first described in 1953 by Palade (Nobel Prize 1974). 

Ricin a toxalbumin phytotoxin from Ricinus seeds. M, 66,000,493 amino acid residues. R. inhibits protein biosynthesis (causes dissociation of polysomes) and has antitumor properties. It consists of an A-chain (M, 32,000) and a B-chain (M, 34,000) joined by disulfide bridges. After reductive separation by 2-mercap-toethanol, both chains show increased inhibitor activity but markedly decreased toxicity. Toxic activity is carried by the A-chain (effectomer), while the B-chain (haptomer) binds the toxin to the cell surface. Similar action and structure are possessed by abrin (M, 65,000 A-chain 30,000, B-chain 35,000), a toxalbumin from the red seeds of Abrus precatorius. Abrin is used in opthalmology. 

Translation in the wider sense equivalent to Protein biosynthesis (see). In the narrower sense, T. is the decoding process whereby each codon (see Genetic code) in mRNA is translated into one of 20 amino acids during protein synthesis on polysomes. 

Fig. 1. Schematic representation of the effect of amino acid supply to the liver on protein synthesis by liver polysomes, and on RNA degradation rate and synthesis of purine nucleotides, The diagram indicates interrelationships between those metabolic events which result in reduced RNA breakdown and increased purine biosynthesis when amino acid supply to the liver is increased (Clifford et al., 1972).

Once the structure of mannan was established to be manno-protein(s) we studied its biosynthesis by following the formation of both the peptide and carbohydrate moieties. In these studies one of the most powerful techniques is the dissection of a metabolic route by inhibiting specific reactions with antimetabolites. In the case of mannoproteins the two moieties are quite different and are polymerized by different biosynthetic routes. We inhibited protein formation with the antibiotic cycloheximide which freezes translation of the genetic message at the level of polysomes. 

In the case of type I, collagen biosynthesis (Fig. 12.20a-h) involves first the synthesis of pro-a - and pro-a -precursor chains. The N-ter-minus of these precursors contains up to 25% of extended a - and -chains (a). Immediately after the chains are released from polysomes, hydroxylation of the proline and lysine residues occurs (cf. reactions under 12.20). 

Fig. 12.20. Collagen biosynthesis (according to Bom-stein, 1974). a Polysome, b hydroxylation, c chain straightening, d disulfide bond formation, e cell membrane, f membrane crossing, g a limited hydrolysis to tropocollagen, h collagen fiber formation, cross-linking...

Ling, V., Dixon, G. H. The biosynthesis of protamine in trout testis. II. Polysome patterns and protein synthetic activities during testis maturation. J. biol. Chem. 245, 3035—3042 (1970). 

These questions must be examined with regard to local circumstances of biosynthesis. In cells, proteins are synthesized either by free polysomes or by rough endoplasmic reticulum (RER) membrane-bound polysomes. For nascent polypeptide chains, these two different situations have to be considered since, in the second case, compartmentalization leads to a vectorial transport and discharge through the membrane. Bound and free polysomes seem to derive from a common precursor pool of ribosomes. Several lines of evidence indicate that membrane-bound ribosomal subunits readily exchange in vivo and no major differences in the rRNA sequences and organization of free and bound ribosomes have been found (Fern and Garlik, 1976 see Shore and Tata, 1977). 

Fig. 1.6. Schematic representation of protein biosynthesis by rough endoplasmic reticulum membrane-bound polysomes (according to Blobel and Dobberstein, 1975a). Codons corresponding to signal peptides after initiation codon AUG are indicated by a zig-zag, the signal sequence is indicated by a dashed line (see text).

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