Cysteine codons

Furthermore, a melanocyte line that has reverted in culture from albino to wild type has restored the cysteine codon at position 85 (Jackson and Bennett, 1990). In vivo studies (see below) proved that the c-locus encodes tyrosinase and showed that expression of a tyrosinase minigene rescues the albino phenotype (Yokoyama et al., 1990 Tanaka et al., 1990 Beermann et al., 1990). 

B. It will be incorporated into a protein in response to a cysteine codon. 

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. 

The hyperthermophiles employ the universal genetic code but some biases are detectable, including preferential use of only two of the six arginine codons and rare usage of cysteine codons. 

B) Alanine would be added each time the cysteine codon was translated. 

Variations in the code for cytoplasmic proteins have been found. In Tetrahymena and other ciliates the codon TAA represents glutamine rather than being a termination codon.375 A few proteins, including some in the human body, contain selenocysteine, the selenium-containing analog of cysteine. Selenocysteine is encoded by termination codon TGA. See Chapter 29 for details. However, even though TGA is occasionally used in this way, it serves as a termination codon for most proteins within the same cells.376 Thus, the context in which the codon TGA occurs determines how it is read by the ribosomal machinery. 

Figure 14.8 The original scheme for oligodeoxynucleotide-directed mutagenesis. The mismatched primer is designed to mutate the codon for cysteine (TGC) to that for serine (AGC).

The unusual amino acid selenocysteine (a derivative of cysteine in which the sulfur atom is replaced by a selenium atom) is an essential component in a small number of proteins. These proteins occur in prokaryotes and eukaryotes ranging from E. coli to humans. In all cases, selenocysteine is incorporated into protein during translation in response to the codon UGA. This codon usually serves as a termination codon but occasionally, in some required but unknown context of bases, is used to specify selenocysteine instead. 

The option to remove the toxin from the environment of susceptible persons is taken in hemochromatosis, a common disorder of iron metabolism(101) that affects 1 in 300 persons of northern European descent. Most cases of hereditary hemochromatosis are attributed to a polymorphism of HFE, which results in substitution of tyrosine for cysteine at codon 282. The disease, untreated, causes liver cirrhosis, heart failure, diabetes and arthritis and leads to early death treatment by phlebotomy to remove excess iron allows affected persons to live a normal life span. 

It is possible to constrain peptides by introducing two codons for cysteine in both sites of the random region. Constraining a peptide in its active conformation is an advantage, but if it is constrained into an incorrect conformation, binding to the target molecule can be prevented. Several peptides have been isolated from cyclic libraries. The screening of pools... 

Cystine is composed of two molecules of cysteine linked through oxidation of their —SH groups to give a disulfide bond. Such oxidation, which is important in stabilizing the folded structure of some proteins, represents a posttranslational modification of a protein. Thus, cystine is never incorporated as such into a polypeptide during translation, and there is no codon that corresponds to it. 

The trace element selenium plays an essential role in the activity of some bacterial and eukaryotic antioxidant enzymes (34). Selenium is incorporated into proteins in the form of the so-called twenty-first amino acid selenocysteine, which is encoded by a UGA stop codon. Although the chemical structure of selenocysteine differs from cysteine only by the replacement of the sulfur atom with selenium, the lower pKa of selenocysteine (5.2) allows for ionization of selenocysteine at physiological pH (35). To read through the UGA stop codon selectively, selenocysteine insertion requires a variety of proteins and RNA stmctures. 

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