0026-8933/00/3406- $25.00 © 2000 MAIK “Nauka / Interperiodica” 0927 Molecular Biology, Vol. 34, No. 6, 2000, pp. 927–933. Translated from Molekulyarnaya Biologiya, Vol. 34, No. 6, 2000, pp. 1081–1089. Original Russian Text Copyright © 2000 by Zvereva, Shpanchenko, Dontsova, Bogdanov. INTRODUCTION Transport/messenger (tm, 10Sa) RNA is a small stable RNA that combines the properties of the corre- sponding individual RNAs. This dual nature provokes particular interest in its structure and function. Although tmRNA has been discovered in the postribo- somal supernatant of Escherichia coli cells more than 20 years ago [1], it was not until four years back that its function became known. The role of tmRNA is to interact with ribosomes stalled on mRNAs that have accidentally lost their stop codon. Such ribosomes are arrested; i.e., they fail to release their mRNA, tRNA, and peptide, and are thereby eecluded from protein synthesis. Upon interaction with tmRNA, translation is switched to its messenger region and a peptide encoded therein is tagged to the incomplete peptide contained in the ribosome. Translation is terminated at the stop codon of the tmRNA coding region, and the peptide thus released is rapidly degraded because its tag contains a recognition site for specific proteases. THE tmRNA GENE In E. coli, tmRNA is encoded by ssrA located at 56.5 min on the genetic map [2]. The gene is monocis- tronic and has its own promoter and terminator, form- ing a single-gene operon [3]. The tmRNA sequence has been established in 50 species by January, 1999 [4]. Most (34) sequences have been extracted from completely sequenced genomes of microorganisms; several others have been revealed via amplification with primers directed to the conserved tRNA-like region of tmRNA [5]. By now, tmRNA has been detected in various eubacteria but not in archaebacte- ria or eukaryotes. The tmRNA genes have been found in the cyanelle genome of Cyanophora paradoxa and in the chloroplast genome of Odontella sinensis [4]. The mitochondrial genome lacks this gene. Though assumed to occur in all bacteria, the gene has not been revealed in the completely sequenced genome of mitochondria-related α-protobacterium Rickettsia prowazekii. The tmRNA gene is necessary for cell growth, since the growth rate is lower in E. coli cells with chromosomal ssrA inactivated by insertion of the chloramphenicol acetyltransferase gene [6]. A signif- icant difference in recovery from carbon starvation has also been observed: a strain carrying nonfunc- tional ssrA recovers more slowly than the wild-type strain [6]. An indirect evidence for the important role of tmRNA is that its gene has been found even in the smallest known bacterial genome of Mycoplasma genitalium [7]. THE tmRNA STRUCTURE The tmRNA size varies from 349 nt in Alcaligenes eutrophus [8] to 411 nt in Mycoplasma capricolum [7]. The E. coli tmRNA is 363 nt [3]. Its secondary struc- ture has been assumed from the phylogenetic analysis of the tmRNA sequences of 50 organisms [5, 9] (Fig. 1). According to the model advanced, tmRNA con- sists of three domains. One includes the 3' and 5' ends of the molecule and forms a tRNA-like structure with an amino acid acceptor stem and a T ΨC hairpin. The other is a single-stranded region and contains a sequence coding for the tag peptide. The third domain joins the tRNA- and mRNA-like domains. This region is highly structured and contains four pseudoknots (Fig. 1). Structure and Function of tmRNA (10Sa RNA) M. E. Zvereva, O. V. Shpanchenko, O. A. Dontsova, and A. A. Bogdanov Chemical Faculty, Moscow State University, Moscow, 119899 Russia; E-mail: dontsova@genebee.msu.su Received May 14, 2000 Abstract—Modern data on the structure and function of transport/messenger (tm) RNA are reviewed. This sta- ble RNA is involved in releasing ribosomes that are unable to complete protein synthesis on mRNA lacking the stop codon. The resulting abnormal proteins are rapidly degraded by specific proteases, which recognize a sig- nal peptide encoded by the template region of tmRNA. The discovery of trans-translation has caused a partic- ular interest in structural and functional studies of tmRNA. Key words: aminoacylation, degradation, RNA, translation, eubacteria UDC 576.85.48