SRNA molecule

Transfer RNA (tRNA) or soluble RNA (sRNA) molecules have been examined at high resolution (Rich, 1977). Transfer RNAs are relatively compact single strands of nucleic acid of 23-30kDa in size that contain 74-94 nucleotides (nt). Individual tRNA transport a covalently attached amino acid to the ribosome and facilitate its proper incorporation into a protein sequence, the latter of which is specified by the sequence of nucleotides read as a triplet code in the mRNA. Thus a tRNA molecule contains two key functional domains ... 

During determination of the sequence of alanine sRNA in the investigation cited above, Holley and co-workers found that during interaction between complementary bases, the molecules of alanine sRNA may acquire several configurations, models of which are represented in Fig. 5. The several unpaired areas evidently determine selective contact of the sRNA molecules, first, with the mRNA template, second, with alanyl-sRNA-synthetase, and third, with the enzyme for transferring sRNA to the template. The functions and biochemistry of transfer RNA are discussed in more detail at the present level of knowledge in surveys by Baev (1965), Kiselev (1964), and Chapeville (1964). 

The model of the mechanism of this interaction assumes that messenger RNA is a special conveyor along which ribosomes move, and each ribosome, because of periodic attachment of sRNA molecules, forms a finished polypeptide chain (Fig. 8) (Zubay, 1963). 

It can thus be concluded that flie chief problems concerned with the mechanism of biosynthesis of proteins and nucleic acids have now been solved. Analysis of this problem has now shifted to the level of filling in all the minor details (discovery of the nucleotide sequence of individual sRNA molecules, the terminal codons, heterogeneity of the aminoacyl-sRNA-synthetases, species specificity of these enzymes for sRNA and for other substrates). The solution of each major problem always brings forth a tremendous number of new problems, and creates a new branch of science. At the same time, however, it provides a decisive stimulus to the analysis of allied problems, sometimes even more important. 

In this case, if leucine codons in the template (or in the DNA cis-tron) are represented, for example, by UUG and CUU only, only the anticodons to them in the sRNA molecule will be active, and anticodons to UUA and CUC will be inactive. If, on the other hand, cistrons in phage DNA are heterogeneous with regard to their leucine codons, and in one particular segment leucine is coded by a codon to which no anticodons are present in the sRNA of E. coli, this segment will be "translated" only when the necessary sRNA... 

According to this proposal, the incorporation of proline into collagen follows the acyl adenylate and acyl RNA stages now generally believed to occur in protein synthesis. A bound hydroxyproline intermediate is postulated, from which hydroxyproline is transferred to soluble RNA. We prefer to suggest that different RNA acceptor molecules exist for hydroxyproline and proline this would be consistent with recent work which indicates that the soluble RNA molecule contains the information for incorporation of a particular amino acid into protein (3). Although it is conceivable that there is hydroxylation of prolyl-sRNA to yield hydroxyprolyl-sRNA, an additional mechanism would be needed... 

Since, as we have seen, all active RNA molecules have the same terminid sequence, pCpCpA, and tince the attachment of the amino acid occurs on the terminal adenosine, the specificity of the RNA molecules must reside in the rest of the chain. Whatever this specificity contists in, however, it does not seem to be species specific, since the activating enzymes from one source can transfer amino acids specifically to RNA s of a very different origin. Hecht and co-workers (188), for example, tiiowed that the soluble fraction from mouse ascites tumor cells could transfer amino acids to RNA prepared from rat liver, calf liver, and yeast as well as to its own RNA. The soluble enzymes from guinea pig liver can transfer amino acids to RNA from rabbit (128) and from E. coli (182), although with the latter the transfer occurs at a slower rate. Furthermore, the activating enzymes and sRNA from liver and Tetrahymena (148), and from liver and spleen (69) have also been used interchangeably. 

From the scheme, we see immediately that the presence of sRNA is essential, since here we have a RNA molecule in actual chemical combination with an activated amino acid, presumably on its way to protein. Furthermore, as we have seen, the formation of the RNA-amino acid compound is extremely sensitive to ribonuclease. In principle, therefore, this step could account for the fact that protein synthesis does not take place in the absence of RNA, and that it is inhibited by ribonuclease. Nevertheless, all evidence to date strongly indicates that the cytoplasmic... 

The reaction of sRNA with formaldehyde was studied by Marini (83) and also by Tissiferes (84)- The latter found the reactivity to be such as to suggest intramolecular hydrogen-bonding between different strands of RNA. Marini found that reaction with formaldehyde, as well as that with nitrous acid and with glyoxal, affected different amino acid acceptor activities to different extents (tyrosine and isoleucine being generally least affected), which indicates that the specificity for different amino acids resides in different nucleotide sequences of the respective RNA molecules. 

In order to establish the point at which the shunt could take place, it would be most important to determine the relationship of the incorporation into particulate protein to the synthesis of specific protein molecules. In the experiments on the synthesis of hemoglobin, fof example, it must be kept in mind that incorporation into undefined, particle-bound protein was obtained by using quite unspecific sRNA, but that for the synthesis of soluble hemoglobin additional, specific factors were required. (For a more detailed discussion of this question see Section IV, B.)... 

Intranuclear synthesis of the three orms of RNA (mRNA, rRNA, and sRNA) which we examined above, is essentially the synthesis of components which interact in the cytoplasm during synthesis of protein molecules. 

According to their theory, as the polycistronic messenger RNA moves in relation to the polysome system, the velocity of protein synthesis in its various parts is slowed. They postulated that the sequence of the genes in the histidine operon (which does not correspond to the biochemical sequence of reactions) is connected with the number of molecules of each enzyme synthesized. By analyzing the frequency of mutations of polarity, they concluded that many triplets (of the 64 possible) can retard the transcription and translation of information. The essence of the matter is that if any nucleotide triplet (codon) XYZ requires an anticodon in the molecules of acceptor sRNA for itstranslationinto a protein "text," a lowered content of this fraction of sRNA with the corresponding anticodon may act as modulator of the velocity of translation, which is reduced at this locus in connection with a decrease in the number of codon-anticodon interactions. 

Weiss and co-workers (1968) have showoi by the method of RNA/DNA hybridization that part of the leucyl and prolyl sRNA formed in E. coli cells after Infection with phage T4 is phage-specific. It may thus be assumed that these phages have genes for the synthesis of a number of phage-specific transfer RNA molecules which are the factors switching cell synthesis over to phage reproduction. 

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