Doing Double Duty

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Science  21 Jul 2000:
Vol. 289, Issue 5478, pp. 361
DOI: 10.1126/science.289.5478.361d

The decoding of genetic information into protein sequence relies upon the interaction of messenger RNA (mRNA), which contains information in the form of a string of three-nucleotide codons, and transfer RNA (tRNA), which contains complementary anticodons at one end and the designated amino acid at the other. In bacteria, a quality control system utilizes a hybrid molecule known as tmRNA that binds to ribosomes lodged on a defective mRNA. The alanine residue of tmRNA accepts the incomplete protein, and the ribosome then switches over to begin translating a portion of the tmRNA that encodes a tag that targets the finished (but defective) protein for proteolytic degradation.

Keiler et al. describe the identification of a circularly permuted, two-piece tmRNA in Caulobacter. This bipartite RNA species is functional, and similar sequences are found in cyanobacteria and Rickettsia and even the mitochondrion of the protist Reclinomonas. Not only does this finding open a new door to studying the mechanisms for translational checkpoints (as reviewed by Karzai et al.), but it serves also to highlight the challenges in annotating permuted sequences. — GJC

Proc. Natl. Acad. Sci. U.S.A.97, 7778 (2000); Nature Struct. Biol.7, 449 (2000).

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