Alanine codons

A. It will be incorporated into a protein in response to an alanine codon. 

C. UCA is a codon for serine. It is converted to a termination codon by mutation A, to a proline codon by mutation B, to a threonine codon by mutation D, and to an alanine codon by mutation E. Only mutation C would produce no change in the protein, since UCU is also a codon for serine. 

The number of mammalian mitochondrial tRNA molecules is 22, which is less than the minimum number (32) needed to translate the universal code. This is possible because in each of the fourfold redundant sets—e.g., the four alanine codons GCU, GCC, GCA, and GCG—only one tRNA molecule (rather than two, as explained above) is used. In each set of four tRNA molecules, the base in the wobble position of the anticodon is U or a modified U (not I). It is not yet known whether this U is base-paired in the codon-anticodon interaction or manages to pair weakly with each of the four possible bases. For those codon sets that are doubly redundant—e.g., the two histidine codons CAU and CAC—the wobble base always forms, a G-U pair, as in the universal code. The structure of the human mitrochondrial tRNA molecule is also different from that of the standard tRNA molecule (except for mitochondrial tRNA UUX). (X = any nucleotide.) The most notable differences are the following ... 

B. Base pairing of alanine codons with anticodon... 

A) Cysteine would be added each time the alanine codon was translated. 

FIGURE 28 12 Translation of mRNA to an ammo acid sequence of a protein starts at an mRNA codon for methionine Nucleophilic acyl substitution transfers the N formylmethionme residue from Its tRNA to the ammo group of the next ammo acid (shown here as alanine) The process converts an ester to an amide... 

X can be any of the four nucleobases—G, A, C or U thus, four anticodons can be formed GGC, GAC, GCC and GUC. The antiparallel structure contains the codons GCC, GUC, GGC and GAC. Today, these codons code for the four amino acids alanine, glycine, valine and aspartic acid. These are, astonishingly, the four protein building blocks produced in the best yields in the Miller-Urey experiment, and they... 

The many (possibly more than 30) types of collagens found in human connective tissues have substantially the same chemical structure consisting mainly of glycine with smaller amounts of proline and some lysine and alanine. In addition, there are two unusual amino acids, hydroxyproline and hydroxylysine, neither of which has a corresponding base-triplet or codon within the genetic code. There is therefore, extensive post-translational modification of the protein by hydroxylation and also by glycosylation reactions. 

Building on earlier work of Osawa and co-workers [55], Oliver and Kowal [52] tested the feasibility of introducing a noncoded amino acid at an unassigned codon in M. luteus. DNA templates were prepared which coded for 19-mer polypeptides containing either the unassigned codon AGA(Arg) or the termination codon TAG at position 13 under the control of a T7 RNA polymerase promoter. The corresponding tRNAs, produced as described in Sect. 2, were based on tRNA and acylated with phenylalanine. The tRNA was modified to prevent recognition by the alanine aminoacyl-tRNA synthetase and to increase translational efficiency. 

The next aminoacyl-tRNA (Figure 14.5 shows a tRNA specific for alanine) is also bound via a codon (GCG)-anticodon (CGC) interaction and is positioned at an adjacent A (for aminoacyl) site on the ribosome. This allows peptide bond formation to... 

When several different codons specify one amino acid, the difference between them usually lies at the third base position (at the 3 end). For example, alanine is coded by the triplets GCU, GCC, GCA, and GCG. The codons for most amino acids can be symbolized by XYq or XYg. The first two letters of each codon are the primary determinants of specificity, a feature that has some interesting consequences. 

Correct answer = B. Once an amino acid is attached to a tRNA molecule, only the anticodon of that tRNA determines the specificity of incor poration. The mischarged alanine will, therefore, be incorporated in the protein at a position determined by a cysteine codon. 

Figure 29-16 Schematic diagram of the tmRNA structure and its function in the rescue of ribosomes stalled at the end of a messenger RNA that has been broken and has lost its inframe termination codon. After it binds into the ribosomal A site the tmRNA, which has been charged with alanine, undergoes the peptidyltransferase reaction and translocation to the P site. Then it lays down its mRNA-like coding sequence, which is used by the ribosome to add ten more amino acids to form the 11-residue C-terminal degradation signal A ANDENYALAA. This induces rapid degradation of the imperfect protein that has been formed.4363...

Each amino acid code consists of a three-base sequence. For example, the DNA sequence CGA codes for the amino acid alanine.6 This sequence in DNA codes for the m-RNA sequence GCU. A three-base pair in m-RNA that codes for an amino acid is called a codon. An anticodon is the three-base sequence in t-RNA that matches a codon in m-RNA. 

Masugi, J., Tamori, Y., Mori, H., Koike, T., and Kasuga, M. (2000). Inhibitory effect of a Proline- to-Alanine Substitution at Codon 12 of Peroxisome Proliferator Activated Receptor y2 on Thiazolidinedione-Induced Adipogenesis. Biochem. Biophys. Res. Commun. 268, 178—182. 

Fig. 8.3 Construction scheme for fragmentary alanine racemase. A termination codon (TAA), a ribosomebinding site (AAGGCAGCGA), and an initiation codon (ATG) were inserted at the sequence corresponding to Thr -Ala-Gln271 of alanine racemase. (Reproduced with permission from Toyama etal., J. Biol. Chem., 266, 13636(1991)).

Fig. 4. Role of the stop codon and lOSa-RNA in E. coli translation. (A) When a stop codon is encountered, a complex of two release factors, RF-1 and RF-3 or RF-2 and RF-3, binds instead of the tRNA. The release factor RF-1 recognizes the stop codons UAA and UAG, while RF-2 recognizes UAA and UGA. The binding of the release factor complex results in hydrolysis of the peptidyl-tRNA and release of the peptide. (B) The role of lOSa-RNA. If truncated mRNA without a stop codon is translated in E. coli, the ribosome stops at the end of the mRNA. lOSa-RNA can then bind to the ribosomal A site and lOSa-RNA can act as tRNA by transferring an alanine to the truncated protein. Subsequently, lOSa-RNA acts as mRNA and a peptide tag with the indicated sequence is added to the truncated protein. lOSa-RNA encodes a stop codon and therefore the protein is released and then degraded by proteases specifically recognizing this C-terminal tag.

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