Pyrrolysine
Encyclopedia : P : PY : PYR : Pyrrolysine
Pyrrolysine is a naturally-occurring genetically-coded amino acid. It is coded by the 'amber' stop codon UAG. Its structure is N6-[(2R,3R)-3-methyl-3,4-dihydro-2H-pyrrol-2-ylcarbonyl]-L-lysine.
Pyrrolysine is a
lysine derivative encoded by the UAG codon in
methylamine methyltransferase
genes of
Methanosarcina barkeri. Near a methyltransferase gene cluster is the
pylT gene, which encodes an unusual transfer RNA (tRNA) with a CUA anticodon. The adjacent
pylS gene encodes a class II aminoacyl-tRNA synthetase that charges the
pylT-derived tRNA with pyrrolysine. The operon containing
pylT and
pylS are also found in the genomes of other sequenced members of the
Methanosarcinaceae family. Homologs of
pylS and
pylT are found in a
Gram-positive bacterium,
Desulfitobacterium hafniense, although the function of these putative genes in this organism is unknown. It was initially shown that
pylT encoded tRNA (CUA) can be charged with lysine by PylS. Recently, it has been shown that the tRNA(CUA) can be charged with lysine
in vitro by the concerted action of the
M. barkeri Class I and Class II Lysyl-tRNA synthetases. Charging a tRNA(CUA) with lysine was originally hypothesized to be the first step in translating UAG amber
codons as pyrrolysine in certain
methanogens. The current model based on
in vitro and
in vivo data favors direct charging of pyrrolysine on to the tRNA(CUA) by the protein product of the
pylS gene. This makes Pyrrolysine the 22nd genetically encoded natural amino acid. The mechanism of encoding makes it the 21st natural directly encoded amino acid.
Pyrrolysine is an amino acid used by some methanogenic (organisms that produce methane) archaea in enzymes that are part of their methane-producing metabolism. It is coded for by a UAG codon, which is normally a stop codon named "amber" but whose meaning might possibly be modified by the presence of a specific downstream sequence, named PYLIS, which forms a stem-loop in the mRNA, forcing the incorporation of pyrrolysine instead of terminating translation. It is also of interest to note that UAG appears to be used as a stop codon very less compared to the other stop codons and whenever it is found in an open reading frame it is always followed shortly by one of more of the other two stop codons.
See also selenocysteine, the 21st genetically encoded amino acid.
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