Leucine codons

Other Systems - Other attenuation-controlled operons of "stalling sequences" include operons for synthesis of leucine, with four adjacent leucine codons in the leader sequence, and of histidine, with... 

According to the wobble principle, what is the minimum number of tRNAs required to decode the six leucine codons—UUA, DUG, GUU, GUC, GUA, and CUG Explain. 

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 ... 

Translation of other synthetic polynucleotides with leucine codons, e.g. poly (A,U) and poly (U,G) (15) is also inhibited in pre-incubated Int. S-10, demonstrating a strong impairment of leucine tENA utilisation. However for the time being, minor effects upon other tENA species, cannot be ruled out. 

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... 

The information contained in the DNA (i.e., the order of the nucleotides) is first transcribed into RNA. The messenger RNA thus formed interacts with the amino-acid-charged tRNA molecules at specific cell organelles, the ribosomes. The loading of the tRNA with the necessary amino acids is carried out with the help of aminoacyl-tRNA synthetases (see Sect. 5.3.2). Each separate amino acid has its own tRNA species, i.e., there must be at least 20 different tRNA molecules in the cells. The tRNAs contain a nucleotide triplet (the anticodon), which interacts with the codon of the mRNA in a Watson-Crick manner. It is clear from the genetic code that the different amino acids have different numbers of codons thus, serine, leucine and arginine each have 6 codewords, while methionine and tryptophan are defined by only one single nucleotide triplet. 

The myopathic form of CPT deficiency is due to a defect of CPT II. The gene for CPT II has been localized to chromosome 1, and several mutations have been identified in patients [4]. As in the case of McArdle s disease (see above), one mutation, a serine-to-leucine substitution at codon 113, is far more common than the others in Caucasians and can be screened for in genomic DNA from blood cells, thus potentially avoiding muscle biopsy. 

RNA codons are three-nucleotide units that either code for specific amino acids in a protein being synthesized or else signal the synthesis to either start or stop. For example, the codon CUA codes for the amino acid leucine. 

Another important technique was based on the observation that synthetic trinucleotides induced the binding to ribosomes of tRNA molecules that were "charged" with their specific amino acids 38/39 For example, the trinucleotides UpUpU and ApApA stimulated the binding to ribosomes of 14C-labeled phenylalanyl-tRNA and lysyl-tRNA, respectively. The corresponding dinucleotides had no effect, an observation that not only verified the two codons but also provided direct evidence for the triplet nature of the genetic code. Another powerful approach was the use of artificial RNA polymers, synthesized by combined chemical and enzymatic approaches.40 For example, the polynucleotide CUCUCUCUCU led to the synthesis by ribosomes of a regular alternating polypeptide of leucine and serine. 

As we have indicated, the codon AUG is the only one generally used to specify methionine, but it serves a dual function in that it is also used to initiate translation. Occasionally, GUG and UUG are also read as an initiating codon in bacteria, but in internal positions these codons are always read as valine and leucine, respectively. In eukaryotes, initiation at codons other than AUG is much less frequent than in prokaryotes. Weak initiation occasionally occurs at GUG, CUG, and ACG codons in eukaryotic systems. The UGA triplet also serves a dual function it is usually recognized as a stop, but on occasion it serves as a codon for selenocys-teine (box 29A). 

Degeneracy of the code is very obvious from an examination of the codon assignments shown in Tabic 17.1. For example, there are six codons for leucine. It should be noted that the nucleotide components of each triplet... 

Would it be possible for a single tRNA to accommodate all the codons for leucine SOLUTION... 

Leucine has six codons, the largest number for any amino acid. According to the wobble hypothesis, a single tRNA can accommodate more than one codon. The wobble hypothesis allows for up to three different nucleotides, but only at the third position in the codon, to interact with a single nucleotide in the anticodon. The fact that there are six codons in leucine means that they must differ at positions other than the third, and they therefore could not be accommodated by a single anticodon in a particular tRNA molecule. 

---