Methanogens translation

The salient features of in vitro translation systems from halophiles, methanogens, and from sulfur-dependent thermophiles belonging to both archaeal kingdoms are summarized below. 

Components of the archaeal translation apparatus, however, display an intra-domain diversity, not encountered within the other two domains. This situation is exemplified by (i) the lack of a uniform compatibility between ribosomes and elongation factors from different archaeal lineages (see section 5.2) (ii) the impressive diversity of archaeal ribosomes and factors in their response to a host of protein synthesis inhibitors (see section 4) (iii) the heterogeneity in shape, mass and composition of archaeal ribosomal subunits (see sections 2.6.2, 2.6.3). Importantly, within-domain diversity is also exemplified by the different complexities of the RNA polymerase subunit patterns from the sulfur-dependent and the methanogenic-halophilic archaea (see Zillig et ah. Chapter 12 of this volume). 

One final interesting amino acid modification is that found in the methanogenic Archaea. These bacteria interpret the amber stop codon as the amino acid methylpyrrolysine, making this the 22" genetically encoded amino acid (the 21 being the N-formyl methionine that eukaryotes use to start translating all their proteins). 

Twenty-two amino acids are naturally incorporated into polypeptides and are called proteinogenic or standard amino acids. Of these, 20 are encoded by the universal genetic code. The remaining 2, selenocysteine and pyrrolysine, are incorporated into proteins by unique synthetic mechanisms. Selenocysteine is incorporated when the mRNA being translated includes a SECIS element, which causes the UGA codon to encode selenocysteine instead of a stop codon. Pyrrolysine is used by some methanogenic archaea in enzymes that they use to produce methane. It is coded for with the codon UAG, which is normally a stop codon in other organisms. 

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