Histidine codons

A ribosome begins to synthesize the leader peptide, but stalls at the histidine codons because it cannot readily find histidine. Because the ribosome is covering up a different part of the mRNA, the message wiU not fold into the correct terminator structure, and RNA polymerase continues transcription through the structural genes of the operon. Translation of the message produces all the enzymes of the histidine biosynthetic pathway. 

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 ... 

Oligonucleotide-mediated mutagenesis was used to change the histidine codon at L173 (CAC) to a leucine codon (CTC). Photosynthetic growth assays indicate that His - Leu does not grow photosynthetically. The absorption spectrum of chromatophores from this mutant suggests that reduced levels of RCs are stably assembled in the membrane, (data not shown). RCs were purified from chromatophore membranes of His ... 

We have also used mutagenic primers to insert histidine tags and epitope tags into cDNAs of interest (Neish et al, 2003). Inserting several codons into a cDNA is a little more difficult than simply substituting one amino acid for another, but is quite feasible using this technique. Optimization of the reaction is absolutely critical when performing more difficult manipulations such as this. 

Figure 1 shows the standard code in DNA language (i. e., as a sequence of triplets in the sense strand of DNA, read in the 5 3 direction see p. 84), represented as a circular diagram. The scheme is read from the inside to the outside. For example, the triplet CAT codes for the amino acid histidine. With the exception of the exchange of U for T, the DNA codons are identical to those of mRNA. 

N-terminal hexahistidine affinity tag. The new vector, pNHis, encodes a protein with anN-terminal extension of six histidine residues followed by six additional amino acids that encode a Factor Xa cleavage site. A stop codon was added to the gene sequence so that the 3 LIC sequence did not add six extra amino acids to the C-terminus of the protein sequence. 

Codons that specify the same amino acid are called synonyms. For example, CAU and CAC are synonyms for histidine. 

Note This table identifies the amino aeid eneoded by each triplet. For example, the codon 5 AUG 3 on mRNA specifies methionine, whereas CAU specifies histidine, UAA, UAG, and UGA are termination signals. AUG is part of the initiation signal, in addition to coding for internal methionine residues. 

Omura F Kohno K, Uchida T (1989) The histidine residue of codon 715 is essential for function of elongation factor 2. EurJ Biochem 180 1-8. 

Codons that specify the same amino acid are called synonyms. For example, CAU and CAC are synonyms for histidine. Note that synonyms are not distributed haphazardly throughout the genetic code (depicted in Table 4.4). In the table, an amino acid specified by two or more synonyms occupies a single box (unless it is specified by more than four synonyms). The amino acids in a box are specified by codons that have the same first two bases but differ in the third base, as exemplified by GUU, GUC, GUA, and GUG. Thus, most synonyms differ only in the last base of the triplet. Inspection of the code shows that XYC and XYU always encode the same amino acid, whereas XYG and XYA usually encode the same amino acid. The structural basis for these equivalences of codons will become evident when we consider the nature of the anticodons of tRNA molecules (Section 30.3). 

For hemoglobin Meoston the tyrosine would come from UAU and UAC codons while the normal hemoglobin s histidine is derived from CAU and CAC. Again we see the substitution of U for C (T for C in the parent DNA). Both are pyrimidines. U (T) usually forms 2 hydrogen bonding pairs while C forms 3. 

Antisense direction CGCGCGGATCCCTACTAATGGTGATG-21 bp sFv gene. Two termination codons will ensure that there will be no readthrough of the gene into the vector. The histidyl residue tag will allow affinity purification by Ni2+-NTA-agarose. Three histidines are routinely incorporated into the RNase-... 

---