发信人: Marble (小石头哥哥), 信区: Biology
标 题: Re: 美科学家在生物体中发现第２２个天然氨基酸
发信站: The unknown SPACE (Wed Jun 12 18:39:28 2002), 站内信件

this paper was published on Science May 24, 296:1459-1462. the
authors are from OSU, Dept. of Microbiology. basically, they
described the finding of genes encoding an unusual tRNA with
CUA anticodon (pylT) and an adjencent gene encoding atypical
aminoacyl-tRNA synthetase (pylS) in Methanosarcina and another
Gram-positive bacteria. these genes are probably cotranscribed.
the tRNA encoded by pylT has unusual 2nd structure that is
different to common tRNA, and the anti-codon is complementary
to the stop codon UAG. as the consequence, the stop codon UAG
is recognized as coding codon and pyrrolysine, a derivative of
lysine is incorporated at this position. pylS, as lysl-tRNA
synthetase, is also special and does not share homology to the
other aminoacyl-transferase. Recombinant pylS can charge lysine
to tRNA, and tRNA(CUA) could be used as the substrate. The
existence of tRNA(CUA) and the unusual cognate lysl-transferase
indicate a mechanism recognizing stop codon UAG as pyrrolysine.
the incorporation could either be cotranslational or before the
tRNA recoginzes the UAG codon. including selenocysteine, pyrrolysine
represents the 22nd GENETICALLY encoded amino acid found in nature.
the question is, is there an enzyme that produces the specific
pyrrolysine? it is possible that some enzyme modify the lysyl-
tRNA into pyrrolysyl-tRNA, either before translation or during
amino acid incorporation in translation. the finding is interesting,
but we can only say that pyrrolysine is the 22nd genetically
encoded amino acid, and we do not know if it exists in the amino
acid pool as natural form.



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