Chapter 8 The Making of tRNAs and More – RNase P and tRNase Z

Transfer RNA instability as a stress response inEscherichia coli: Rapid dynamics of the tRNA pool as a function of demand

“…The E. coli genome encodes more than 20 ribonucleases, including a number that participate in the processing of precursor transcripts to generate mature tRNAs, such as RNase E, RNase P, RNase PH and RNase T (reviewed in refs. 32,33 ). However, mature functional tRNAs are normally not considered substrates for any of the cytoplasmic RNases, due to the protective qualities of their compact tertiary structure, their aminoacylated 3′-ends, and their associations with cellular protein partners (reviewed in refs.…”

Repairing tRNA termini: News from the 3' end

“…[8][9][10] The only known exception is the parasitic archaeon Nanoarchaeum equitans, where tRNAs are transcribed without 5 0 extensions and do thereby not require any further 5 0 processing. 11 However, in many genetic systems (e.g., bacteria, mitochondria, chloroplasts) tRNAs are even embedded in polycistronic transcripts together with other RNA species.…”

“…11 However, in many genetic systems (e.g., bacteria, mitochondria, chloroplasts) tRNAs are even embedded in polycistronic transcripts together with other RNA species. 8,10,12 The precise release of the tRNA sequence from the different precursors by specific nucleases is thus an essential and generally early step in tRNA biogenesis.…”

“…The 5 0 leader of tRNA precursors is universally removed by an endonuclease called RNase P. 8,[13][14][15][16] The enzyme is found in one of 2 fundamentally different forms: either as a ribonucleoprotein based on a catalytic RNA molecule, or as a pure protein enzyme; 8,[13][14][15][16] both nevertheless function in an essentially similar way and are experimentally exchangeable. 17 With the exception of some tRNAs His in Eukarya (see below), RNase P forms the final, mature 5 0 end of tRNAs by a precise endonucleolytic cut.…”

“…Here, often endo-as well as exonucleases are involved, either alternatively or in combination. [8][9][10]12,18,19 The major 3 0 -end processing endonuclease RNase Z (also called tRNase Z) functions by precisely clipping off 3 0 extensions following the discriminator base, similar to RNase P. 8,19,20 Consequently, in bacteria, where the 3 0 -terminal CCA is frequently encoded by tRNA genes, a different, multistep process involving the concerted action of RNase E and a handful of (in part redundant) exonucleases, is finally trimming the extension down to the mature CCA end; 8,10,[21][22][23][24] in some cases, RNase E directly generates the mature CCA terminus rather than cleaving downstream. 25 Nevertheless, the RNase Z of Thermotoga maritima has adapted to also cleave downstream of the CCA, thereby maturing the 3 0 end of CCA-containing precursors in a onestep process.…”