Two Many Synthetases

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Science  29 Aug 2003:
Vol. 301, Issue 5637, pp. 1161
DOI: 10.1126/science.301.5637.1161a

Aminoacyl transfer RNA (tRNA) synthetases (aaRSs) are at the heart of the transition from an ancient RNA-controlled economy to today's community of autonomous protein agents. These enzymes attach amino acids to their cognate tRNAs, and most organisms possess the canonical set of 20 synthetases, which fall into the structurally distinct classes I and II.

Roy et al. studied the archaeon Pyrococcus abyssi, which contains the full-length AsnRS as well as AsnRS2, which lacks the tRNA-binding domain. They show that AsnRS2 synthesizes asparagine from aspartate, ATP, and ammonia; phylogenetic analysis suggests that the tRNA-binding domain was lost, probably as a protective measure during the genesis of the free-standing biosynthetic enzyme from the ancestral AspRS. This path (AsnRS2 + AsnRS) to making Asn-tRNAasn complements the indirect archaeal route of using a relaxed-specificity AspRS to make Asp-tRNAasn followed by amidotransferase modification of the tRNA-bound amino acid.

Polycarpo et al. examined one of the very few cases in which both class I and class II aaRSs are found in a single organism, the archaeon Methanosarcina barkeri. In addition to charging the usual suspects (tRNAlysCUU and tRNAlysUUU), LysRS1 and LysRS2 cooperate to attach lysine to tRNApyl, a recently discovered component believed to insert the 22nd amino acid pyrrolysine cotranslationally. While LysRS2 carries out the chemical steps, LysRS1 may function as chaperone. These authors suggest that the aaRS-like PylS (which also seems to lack a tRNA-binding domain) may perform an in situ conversion of the attached lysine into pyrrolysine. — GJC

Proc. Natl. Acad. Sci. U.S.A. 100, 9837 (2003); Mol. Cell, in press.

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