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Science  22 Feb 2002:
Vol. 295, Issue 5559, pp. 1427-1429
DOI: 10.1126/science.295.5559.1427d

The assembly-line production of proteins takes place in the factory known as the ribosome. The amino acids (attached to their corresponding transfer RNAs) are recruited in a precise order that is specified by the order of the triplet-nucleotide codons in the messenger RNA (mRNA) instruction guide. Termination codons signal a halt to production; the factory is then disassembled and recycled for use at the beginning of another mRNA.

Several recent crystal structures of release and recycling factors (eRF1, RF2, and RRF) have suggested that they mimic the overall shape of transfer RNA, and a critical three-amino acid motif (GGQ) in the release factors has been shown to mediate recognition of the termination codons. Nakahigashi et al. and Heurgué-Hamard et al. now identify HemK as the enzyme that methylates the glutamine (Q), yielding N-5-methylglutamine. Mutations in hemK result in severe growth defects as well as termination inefficiency. What makes this particularly unexpected is the history of the hemK gene: first as a suppressor discovered in a screen for heme biosynthetic enzymes (hence its name) and later as a sequenced gene exhibiting similarity to a family of adenine-specific DNA methyltransferases. — GJC

Proc. Natl. Acad. Sci. U.S.A.99, 1473 (2002); EMBO J., 21, 769 (2002).

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