Isoacceptor tRNAs

Recognition of cognate tRNAs. Many attempts have been made to learn what part or parts of tRNA molecules are involved in recognition by aminoacyl-tRNA synthetases. Nucleotide sequences of isoacceptor tRNAs have been compared. Chemically modified and fragmented tRNA molecules have been studied, and many mutant tRNAs have been made. These... 

Some data suggested that a transient covalent linkage of tRNA to the synthetases may form through addition of a nucleophilic group of the enzyme to the 6 position of the uracil (or 4-thiouracil) present in position 8 of all tRNAs (Eq. 29-3). The two isoacceptors tRNA y species in E. coli contain 4-thiouracil at this position. The C=C bond in this base can be saturated by sodium borohydride reduction, which was found not only to prevent the covalent interaction with the enzyme but also to prevent aminoacylation of the tRNA. However, Eq. 29-3 probably describes a side reaction irrelevant to tRNA function. 

Oxidation of Sephadex G-50 or Enzacryl polyacetal using periodate affords aldehydic pol5nners with which the primary amino groups of the amino acid moieties in aminoacyl-tRNA form Schiff bases, and coupling is rendered irreversible by reduction with sodium cyanoborohydride. Hence, only the amino-acylated tRNA molecules in a mixture are retained, and the tRNA may subsequently be released by washing with ammonium bicarbonate buffer, thus affording a convenient method for the separation of isoacceptor tRNA species. ... 

Each tRNA is specific for only one particular amino acid. However, the triplet code displays degeneracy and consequently there are often several different tRNA molecules (isoacceptor tRNAs (Sprinzl et al, 1991)) for one particular amino acid, each with a different three-base anticodon. Surprisingly, even though aU tRNA molecules are believed to adopt similar tertiary structures, the enzymatic attachment of the correct amino acid on to its cognate tRNA by an aminoacyl-tRNA synthetase is carried out with profound fidelity (Schimmel, 1987). Of particular interest is the determination of those features of a tRNA molecule that lead to the attachment of the proper amino acid. 

Derwenskus, K. H., Fischer, W., and Sprinzl, M. (1984). Isolation of tRNA isoacceptors by affinity chromatography on immobilized bacterial elongation factor Tu. Anal. Biochem. 136, 161-167. 

The product of the selC gene, tRNA =, was identified as a novel tRNA species. Its structure differs from the features of all elongator tRNAs. With 95 nucleotides, it is the longest tRNA known to date. It has a variable loop of 22 nucleotides, an eight-base-pair aminoacyl acceptor stem, and a number of deviations from tRNA consensus structure. tRNA is charged with L-serine by the cellular seryl-tRNA synthetase, which is responsible for aminoacylation of the serine isoacceptor species. Thus, selenocysteine is formed from a serine residue charged to tRNA =. 

Aminoacyl-tRNA synthetases (aaRSs) are critical components of the translation machinery for protein synthesis in every living cell (1). Each aaRS enzyme in this family links a single amino acid covalently to one or more tRNA isoacceptors to form charged tRNAs. Identity elements within the tRNAs serve as molecular determinants or antideterminants that aid in selection by cognate aaRSs (2). Some aaRSs also have an amino acid editing mechanism to clear their mistakes (3). The canonical aaRSs and aaRS-like proteins have functionally diverged to perform many other important roles in the cell (4, 5). Their versatility and adaptability have provided unique opportunities to develop biotechnology tools and to advance medical research. 

EF-Tu will bind to any aminoacylated tRNA other than tRNA the initiator tRNA (step c. Fig. 29-12), and carry it to the ribosome (step d), where it binds into the A site. There it is selected if it forms a proper base pair with the mRNA codon in the A site or is rejected if it does not. This decoding process involves both an initial step and a proofreading step. The aminoacyl-tRNA binds both to the decoding site in the 16S RNA and to the peptidyltransferase site in the 23S RNA. (See discussions on p. 1687.) The decoding site is on the platform at the upper end of helix 44 (Fig. 29-2). Nucleotide G1401 plays a crucial roie.375 vVhen one of the isoacceptor species of E. coU tRNA is irradiated with ultraviolet light, the... 

Homozygotes su(s) /su(s) showed an inhibition of the production of the negative effector and therefore activated the suppressible vermilion allele. The absence in su(s) /su(s) individuals of an isoacceptoral form of tyrosine tRNA may possibly play the role of the negative effector (Baillie and Chovnick, 1971 Twardzik et al., 1971). This hypothesis was developed by White and his co-workers (White et al., 1973). 

As has been mentioned, the su(s) locus also suppressed alleles of the purple, speck, and sable loci. In this case it is possible that the reduction in content of tRNA P, tRNA P, tRNA , and tRNA during Drosophila development is responsible for suppression. Jacobson et al., (1975) suggested that the su(s) locus produced an enzyme which modified the second isoacceptor of tRNA , and when such modification is absent [e.g., in the su(s) mutant], tRNA is not able to accept tyrosine from one of the fractions of tyrosyl-tRNA ligase. 

A The special problem of the particular methionine tRNA (tRNA ) that, once aminoacylated to give Met-tRNA, can be formylated to Met-tRNA may be solved by the use of a subscript f (in the isoacceptor position) or by the use of tRNA . us tRNA (or tRNA ) can be converted enzymically to Met-tRNA (or Met-tRNA ) and then to fMet-ttoA (or fMet-tRNA ) Met-tRNA cannot be formylated enzymically. 

---