Purification and Properties of Ribonuclease II from Escherichia coli

Genome Sequence and Analysis of Escherichia coli MRE600, a Colicinogenic, Nonmotile Strain that Lacks RNase I and the Type I Methyltransferase, EcoKI

Escherichia coli strain MRE600 was originally identified for its low RNase I activity and has therefore been widely adopted by the biomedical research community as a preferred source for the expression and purification of transfer RNAs and ribosomes. Despite its widespread use, surprisingly little information about its genome or genetic content exists. Here, we present the first de novo assembly and description of the MRE600 genome and epigenome. To provide context to these studies of MRE600, we include comparative analyses with E. coli K-12 MG1655 (K12). Pacific Biosciences Single Molecule, Real-Time sequencing reads were assembled into one large chromosome (4.83 Mb) and three smaller plasmids (89.1, 56.9, and 7.1 kb). Interestingly, the 7.1-kb plasmid possesses genes encoding a colicin E1 protein and its associated immunity protein. The MRE600 genome has a G + C content of 50.8% and contains a total of 5,181 genes, including 4,913 protein-encoding genes and 268 RNA genes. We identified 41,469 modified DNA bases (0.83% of total) and found that MRE600 lacks the gene for type I methyltransferase, EcoKI. Phylogenetic, taxonomic, and genetic analyses demonstrate that MRE600 is a divergent E. coli strain that displays features of the closely related genus, Shigella. Nevertheless, comparative analyses between MRE600 and E. coli K12 show that these two strains exhibit nearly identical ribosomal proteins, ribosomal RNAs, and highly homologous tRNA species. Substantiating prior suggestions that MRE600 lacks RNase I activity, the RNase I-encoding gene, rna, contains a single premature stop codon early in its open-reading frame.

Transition Metal Homeostasis

This chapter focuses on transition metals. All transition metal cations are toxic-those that are essential for Escherichia coli and belong to the first transition period of the periodic system of the element and also the "toxic-only" metals with higher atomic numbers. Common themes are visible in the metabolism of these ions. First, there is transport. High-rate but low-affinity uptake systems provide a variety of cations and anions to the cells. Control of the respective systems seems to be mainly through regulation of transport activity (flux control), with control of gene expression playing only a minor role. If these systems do not provide sufficient amounts of a needed ion to the cell, genes for ATP-hydrolyzing high-affinity but low-rate uptake systems are induced, e.g., ABC transport systems or P-type ATPases. On the other hand, if the amount of an ion is in surplus, genes for efflux systems are induced. By combining different kinds of uptake and efflux systems with regulation at the levels of gene expression and transport activity, the concentration of a single ion in the cytoplasm and the composition of the cellular ion "bouquet" can be rapidly adjusted and carefully controlled. The toxicity threshold of an ion is defined by its ability to produce radicals (copper, iron, chromate), to bind to sulfide and thiol groups (copper, zinc, all cations of the second and third transition period), or to interfere with the metabolism of other ions. Iron poses an exceptional metabolic problem due its metabolic importance and the low solubility of Fe(III) compounds, combined with the ability to cause dangerous Fenton reactions. This dilemma for the cells led to the evolution of sophisticated multi-channel iron uptake and storage pathways to prevent the occurrence of unbound iron in the cytoplasm. Toxic metals like Cd2+ bind to thiols and sulfide, preventing assembly of iron complexes and releasing the metal from iron-sulfur clusters. In the unique case of mercury, the cation can be reduced to the volatile metallic form. Interference of nickel and cobalt with iron is prevented by the low abundance of these metals in the cytoplasm and their sequestration by metal chaperones, in the case of nickel, or by B12 and its derivatives, in the case of cobalt. The most dangerous metal, copper, catalyzes Fenton-like reactions, binds to thiol groups, and interferes with iron metabolism. E. coli solves this problem probably by preventing copper uptake, combined with rapid efflux if the metal happens to enter the cytoplasm.

Post-transcriptional control of gene expression: bacterial mRNA degradation

Many biological processes cannot be fully understood without detailed knowledge of RNA metabolism. The continuous breakdown and resynthesis of prokaryotic mRNA permit rapid production of new kinds of proteins. In this way, mRNA levels can regulate protein synthesis and cellular growth. Analysing mRNA degradation in prokaryotes has been particularly difficult because most mRNA undergo rapid exponential decay. Prokaryotic mRNAs differ in their susceptibility to degradation by endonucleases and exonucleases, possibly because of variation in their sequencing and structure. In spite of numerous studies, details of mRNA degradation are still largely unknown. This review highlights those aspects of mRNA metabolism which seem most influential in the regulation of gene expression.

Genomic analysis in Escherichia coli demonstrates differential roles for polynucleotide phosphorylase and RNase II in mRNA abundance and decay

Previous work has shown that simultaneous inactivation of polynucleotide phosphorylase (PNPase) and RNase II (both 3' 5' exonucleases) in Escherichia coli leads to the loss of cell viability and the accumulation of partially degraded mRNA species. In order to help to distinguish how these two enzymes globally affect the abundance and decay of mRNAs, we have carried out a genome-wide analysis of the steady-state levels of E. coli transcripts using deletion mutations in either rnb or pnp. The data show that, in exponentially growing cells, inactivation of PNPase leads to an increase in the steady-state level of more expressed mRNAs (17.3%) than inactivation of RNase II (7.3%). In contrast, the steady-state levels of a large number of E. coli mRNAs (31%) are decreased in the absence of RNase II, including almost all the ribosomal protein genes, suggesting that a major function of this enzyme is to protect specific mRNAs from the activity of other ribonucleases. Array data were confirmed by Northern analysis of 12 individual mRNAs. A comparison between the steady-state levels and the half-lives of individual mRNAs indicates that there may be a direct interaction between transcription and mRNA decay for some of the transcripts. In addition, results are presented to show significant phenotypic differences between the pnp-7 point mutant and the pnp delta 683 deletion allele.

The reaction mechanism of ribonuclease II and its interaction with nucleic acid secondary structures

Ribonuclease II is a processive 3'- to 5'-exoribonuclease in Escherichia coli with two binding sites: a catalytic site associated with the first few 3'-nucleotides and an anchor site binding nucleotides approximately 15 to 25 from the 3'-end. When RNase II degrades single-stranded helical poly(C), the enzyme-substrate complex dissociates at discrete intervals of 12 nucleotides. RNase II stalled at the last rC of single-stranded 3'-(rC)(n)(dC)(m) oligonucleotides. The more residues released, the faster the stalled complex dissociated and the less it inhibited RNase II activity, i.e. the enzyme-substrate association weakened progressively. Using phosphodiesterase I (PDE I) as a probe, a method was developed to identify cytidine residues in (32)P-oligonucleotides interacting with a protein. PAGE bands corresponding to nucleotides 1-6 from the 3'-end were consistent with interaction at the catalytic site, and following a gap, bands approximately 15 to 25 from the 3'-end, with anchor site association. Both 3' and 5' binding were necessary to maintain the complex. Of most significance, the original anchor site nucleotides remained fixed at the anchor site while the 3'-end was pulled, or threaded, through the catalytic site, i.e. the substrate did not 'slide' through the enzyme. DNA oligonucleotides with double-stranded stem-loops were good competitive inhibitors of RNase II. A 3'-single-stranded arm was essential, while optimal binding required both 5'- and 3'-arms. PDE I probing indicated that the nucleotides at the anchor site were specified by the spatial distance from the catalytic site, and on only one of the duplex strands. When degradation of a structured RNA paused or stopped, the RNase II-product commenced cycles of dissociation-reassociation. Duplex strand binding by RNase II made complex DNA or RNA structures accessible to degradation by other nucleases and further verified the PDE I footprinting method.

Degradation of mRNA in Escherichia coli: an old problem with some new twists

Metabolic instability is a hallmark property of mRNAs in most if not all organisms and plays an essential role in facilitating rapid responses to regulatory cues. This article provides a critical examination of recent progress in the enzymology of mRNA decay in Escherichia coli, focusing on six major enzymes: RNase III, RNase E, polynucleotide phosphorylase, RNase II, poly(A) polymerase(s), and RNA helicase(s). The first major advance in our thinking about mechanisms of RNA decay has been catalyzed by the possibility that mRNA decay is orchestrated by a multicomponent mRNA-protein complex (the "degradosome"). The ramifications of this discovery are discussed and developed into mRNA decay models that integrate the properties of the ribonucleases and their associated proteins, the role of RNA structure in determining the susceptibility of an RNA to decay, and some of the known kinetic features of mRNA decay. These models propose that mRNA decay is a vectorial process initiated primarily at or near the 5' terminus of susceptible mRNAs and propagated by successive endonucleolytic cleavages catalyzed by RNase E in the degradosome. It seems likely that the degradosome can be tethered to its substrate, either physically or kinetically through a preference for monphosphorylated RNAs, accounting for the usual "all or none" nature of mRNA decay. A second recent advance in our thinking about mRNA decay is the rediscovery of polyadenylated mRNA in bacteria. Models are provided to account for the role of polyadenylation in facilitating the 3' exonucleolytic degradation of structured RNAs. Finally, we have reviewed the documented properties of several well-studied paradigms for mRNA decay in E. coli. We interpret the published data in light of our models and the properties of the degradosome. It seems likely that the study of mRNA decay is about to enter a phase in which research will focus on the structural basis for recognition of cleavage sites, on catalytic mechanisms, and on regulation of mRNA decay.

Overexpression, purification, and properties of Escherichia coli ribonuclease II

Ribonuclease II (RNase II) is a major exonuclease in Escherichia coli that hydrolyzes single-stranded polyribonucleotides processively in the 3' to 5' direction. To understand the role of RNase II in the decay of messenger RNA, a strain overexpressing the rnb gene was constructed. Induction resulted in a 300-fold increase in RNase II activity in crude extracts prepared from the overexpressing strain compared to that of a non-overexpressing strain. The recombinant polypeptide (Rnb) was purified to apparent homogeneity in a rapid, simple procedure using conventional chromatographic techniques and/or fast protein liquid chromatography to a final specific activity of 4,100 units/mg. Additionally, a truncated Rnb polypeptide was purified, solubilized, and successfully renatured from inclusion bodies. The recombinant Rnb polypeptide was active against both [3H]poly(A) as well as a novel (synthetic partial duplex) RNA substrate. The data show that the Rnb polypeptide can disengage from its substrate upon stalling at a region of secondary structure and reassociate with a new free 3'-end. The stalled substrate formed by the dissociation event cannot compete for the Rnb polypeptide, demonstrating that duplexed RNAs lacking 10 protruding unpaired nucleotides are not substrates for RNase II. In addition, RNA that has been previously trimmed back to a region of secondary structure with purified Rnb polypeptide is not a substrate for polynucleotide phosphorylase-like activity in crude extracts. The implications for mRNA degradation and the proposed role for RNase II as a repressor of degradation are discussed.

Construction and characterization of an absolute deletion mutant of Escherichia coli ribonuclease II

The degradation of mRNA plays a central role in the control of protein synthesis. In Escherichia coli, the rnb gene encodes ribonuclease II (RNase II), one of the two main exonucleases involved in mRNA decay. We have constructed strain CMA201, in which the rnb promoter region and the gene were deleted from the chromosome and replaced by a tetr cassette. This is the first rnb absolute deletion mutant that shows the complete absence of rnb-specific mRNA. This strain has growth characteristics similar to the wild-type, even though it has no RNase II activity, and it should be useful in studies of mRNA metabolism.

Phosphorolytic error correction during transcription

Escherichia coli DNA-directed RNA polymerase is shown to contain a novel phosphorolytic error correction activity which removes erroneous nucleotides, as rNDPs, from the 3'-end of the growing transcript. The activity we describe is biochemically similar to polynucleotide phosphorylase (PNP), yet in contrast to PNP is activated by Mn2+. We demonstrate that the activity, which is mediated by Pi, is dependent on the presence of an incorrectly incorporated nucleotide at the leading 3'-end of the transcript. The correction activity we describe exhibits a 4 x 10(4)-fold preference for the excision of incorrect nucleotides from the transcript. These findings suggest the possibility that RNA phosphorolysis may play a critical role in the process of transcriptional proofreading.

DNA sequencing and expression of the gene rnb encoding Escherichia coli ribonuclease II

The Escherichia coli ribonuclease II (RNase II) is an exonuclease involved in mRNA degradation that hydrolyses single-stranded polyribonucleotides processively in the 3' to 5' direction. Sequencing of a 2.2 kb MseI-RsaI fragment containing the rnb gene revealed an open reading frame of 1794 nucleotides that encodes a protein of 598 amino acid residues, whose calculated molecular mass is 67,583 Da. This value is in good agreement with that obtained by sodium dodecyl sulphate/polyacrylamide gel electrophoresis of polypeptides synthesized by expression with the T7 RNA polymerase/promoter system. This system was also used to confirm the correct orientation of rnb. Translation initiation was confirmed by rnb-lacZ fusions. The mRNA start site was determined by S1 nuclease mapping. Two E. coli mutants harbouring different rnb alleles deficient in RNase II activity were complemented with the expressed fragment carrying the rnb gene.

Analysis of mRNA decay and rRNA processing in Escherichia coli multiple mutants carrying a deletion in RNase III

RNase III is an endonuclease involved in processing both rRNA and certain mRNAs. To help determine whether RNase III (rnc) is required for general mRNA turnover in Escherichia coli, we have created a deletion-insertion mutation (delta rnc-38) in the structural gene. In addition, a series of multiple mutant strains containing deficiencies in RNase II (rnb-500), polynucleotide phosphorylase (pnp-7 or pnp-200), RNase E (rne-1 or rne-3071), and RNase III (delta rnc-38) were constructed. The delta rnc-38 single mutant was viable and led to the accumulation of 30S rRNA precursors, as has been previously observed with the rnc-105 allele (P. Gegenheimer, N. Watson, and D. Apirion, J. Biol. Chem. 252:3064-3073, 1977). In the multiple mutant strains, the presence of the delta rnc-38 allele resulted in the more rapid decay of pulse-labeled RNA but did not suppress conditional lethality, suggesting that the lethality associated with altered mRNA turnover may be due to the stabilization of specific mRNAs. In addition, these results indicate that RNase III is probably not required for general mRNA decay. Of particular interest was the observation that the delta rnc-38 rne-1 double mutant did not accumulate 30S rRNA precursors at 30 degrees C, while the delta rnc-38 rne-3071 double mutant did. Possible explanations of these results are discussed.

RNase I*, a form of RNase I, and mRNA degradation in Escherichia coli

A previously unreported endoRNase present in the spheroplast fraction of Escherichia coli degraded homoribopolymers and small RNA oligonucleotides but not polymer RNA. Like the periplasmic endoRNase, RNase I, the enzyme cleaved the phosphodiester bond between any nucleotides; however, RNase I degraded polymer RNA as fast as homopolymers or oligomers. Both enzymes migrated as 27-kDa polypeptides by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and could not be separated by various chromatographic procedures. In rna insertion mutants, both enzymes were completely missing; the spheroplast enzyme is called RNase I*, since it must be a form of RNase I. The two forms could be distinguished by physical treatments. RNase I could be activated by Zn2+, while RNase I* was inactive in the presence of Zn2+. RNase I was inactivated very slowly at 100 degrees C over a wide pH range, while RNase I* was inactivated slowly by heat at pH 4.0 but much more rapidly as the pH was increased to 8.0. In the presence of a thiol-binding agent, the inactivation at the higher pH values was much slower. These results suggest that RNase I*, but not RNase I, has free sulfhydryl groups. RNase I* activity in the cell against a common substrate was estimated to be several times that of RNase I. All four 2',3'-phosphomonoribonucleotides were identified in the soluble pools of growing cells. Such degradative products must arise from RNase I* activity. The activity would be suited for the terminal step in mRNA degradation, the elimination of the final oligonucleotide fragments, without jeopardizing the cell RNA. An enzyme with very similar specificity was found in Saccharomyces cerevisiae, suggesting that the activity may be widespread in nature.

RNase III cleavages in non-coding leaders of Escherichia coli transcripts control mRNA stability and genetic expression

The primary transcripts of the rpsO-pnp, rnc-era-recO and metY-nusA-infB operons of E coli are each processed by RNase III, upstream of the first translated gene, in hair-pin structures formed by the 5' non-coding leader. The mRNAs of the 3 operons, of which the 5' terminal motifs have been removed by RNase III, decay significantly more rapidly than the uncut transcripts which accumulate in the RNase III deficient strain. The rapid decay of a primary transcript of the metY-nusA-infB operon, initiated at a secondary promoter in the vicinity of the RNase III sites, suggests that the 5' features upstream of the RNase III cutting sites are responsible for the stability of the uncut RNAs. RNase III autocontrols its own expression by removing the 5' motif which stabilizes its mRNA. Similarly, the synthesis of polynucleotide phosphorylase and of protein Era are also controlled by RNase III cleavages which trigger the degradation of their messengers. The role of RNase III in the regulation of gene expression and the possible mechanisms of mRNA stabilization and of 5' to 3' decay initiated by RNase III processing are discussed.

Stabilization of discrete mRNA breakdown products in ams pnp rnb multiple mutants of Escherichia coli K-12

The degradation of mRNA in Escherichia coli is thought to occur through a series of endonucleolytic and exonucleolytic steps. By constructing a series of multiple mutants containing the pnp-7 (polynucleotide phosphorylase), rnb-500 (RNase II), and ams-1 (altered message stability) alleles, it was possible to study general mRNA turnover as well as the degradation of specific mRNAs. Of most interest was the ams-1 pnp-7 rnb-500 triple mutant in which the half-life of total pulse-labeled RNA increased three- to fourfold at the nonpermissive temperature. RNA-DNA hybridization analysis of several specific mRNAs such as trxA (thioredoxin), ssb (single-stranded-DNA-binding protein), uvrD (DNA helicase II), cat (chloramphenicol acetyltransferase), nusA (N utilization substance), and pnp (polynucleotide phosphorylase) demonstrated two- to fourfold increases in their chemical half-lives. A new method for high-resolution Northern (RNA) analysis showed that the trxA and cat mRNAs are degraded into discrete fragments which are significantly stabilized only in the triple mutant. A model for mRNA turnover is discussed.

The 3' end of drosophila histone H3 mRNA is produced by a processing activity in vitro

We have examined the process by which the 3' terminus of the Drosophila histone H3 mRNA is produced in vitro. When a template containing a portion of DNA that flanks the normal 3' end of the histone H3 gene and an oligo dC tail on the template strand is transcribed in vitro by Drosophila RNA polymerase II, transcription continues beyond the 3' end of the H3 gene. A processing activity was identified that cleaves the precursor transcript generating an RNA species with the same 3' end as the mature H3 mRNA. The processing activity was partially purified by ion exchange chromatography and sucrose gradient sedimentation. The isolated activity was found to require Mg++ but did not require addition of a nucleoside triphosphate for activity. The activity sedimented with a molecular weight of approximately 140,000 daltons. Transcription of the template and processing of the RNA can be uncoupled in vitro.

Degradation of ribosomal RNA in bacteriophage lambda lysogens after thermal induction

Stable RNA of Escherichia coli was extensively degraded about 40 min after thermal induction of lysogenized lambda cI857 phages at 42 degrees C. When several nuclease-deficient host cells were tested, RNase I activity in the host cells was inferred to be involved in the RNA degradation. Ribosomal structure was detectably altered before the degradation of ribosomal RNA was observed. 30S and 50S subunits began to sediment at 25-28S and 45-58S, respectively, still containing intact RNA. Nonpermissive host cells lysogenized with lambda cI857 susR produced progeny phages in normal burst size after thermal induction and then degraded stable RNA, though they were not lysed. In contrast cells lysogenized with lambda cI857 susS produced ten times more progeny phages under the same condition, but did not degrade stable RNA. These results indicate that the lambda S gene product, which acts as a positive effector of lysis, induced the degradation of stable RNA, presumably by a still uncharacterized effect on the cytoplasmic membrane.

Antagonistic action between antibodies directed against 7-methylguanosine and polyamines on translation in vitro of RNA induced by measles virus

Antibodies directed against N7-methylguanosine (m7Guo) were prepared and added to a wheat germ cell-free protein-synthesizing system programmed with RNA extracted from monkey cells persistently infected with measles virus. A dose-dependent inhibition of [35S]methionine incorporation was observed when RNA was preincubated with anti-m7Guo immunoglobulins. Antibodies preincubated with m7Guo did not show any inhibiting activity. The inhibitory effect of antibodies was abolished when RNA was preincubated with immunoglobulins in the presence of spermine and spermidine. When polyamines were added to the assay programmed with the IgG-RNA complex, no inhibition was observed.

Altered bacteriophage T4 ribonucleic acid metabolism in a ribonuclease II-deficient mutant of Escherichia coli

Some early T4 ribonucleic acids were not found in an infected ribonuclease II-deficient strain but were formed in ribonuclease II+ transductants as well as in wild-type Escherichia coli.

A microsomal exoribonuclease from rat liver

A exoribonuclease has been purified from the microsomes of rat liver. The enzyme had an apparent molecular weight of 80 000-83 000 and produced, via a processive mechanism, 5'-AMP as the only product from poly(A). The degradation was found to proceed in the 3' to 5' direction. The relative rates of breakdown of synthetic polynucleotides by the enzyme under standard conditions were in the order poly(A) equal to poly(U) greater than poly(C). In addition to Mg2+, K+ was required for maximum activity. The enzymic activity was inhibited by p-chloromercuribenzoate and poly(G), but not by a rate liver RNAase inhibitor. The effect of spermine on the breakdown of synthetic polynucleotides by the enzyme has been studied. In the absence of K+, the breakdown of poly(C) was stimulated and that of poly(A) was stimulated slightly. However, the breakdown of poly(U) was inhibited slightly by spermine.

Purification of RNAase II by preparative polyacrylamide gel electrophoresis

Purification of RNAase II to electrophoretic homogeneity is described. The exonuclease is activated by K+ and Mg2+ and hydrolyses poly(A) to 5'-AMP, exclusively as described by Nossal and Singer (1968, J. Biol. Chem. 243, 913--922). To separate RNAase II from ribosomes, DEAE-cellulose chromatography was used. Two additional chromatographic steps give a preparation that yields 10 bands after analytical polyacrylamide gel electrophoresis. Preparative polyacrylamide gel electrophoresis resulted in a final preparation which on analytical polyacrylamide gels gives a single band. A molecular weight of 76 000 +/- 4000 was obtained from Sephadex G-200 chromatography, with three bands from sodium dodecyl sulfate (SDS) denaturation and SDS gel electrophoresis. The subunits have a molecular weight of 40 000 +/- 2000, 33 000 +/- 2000, and 26 000 +/- 1000. The enzyme thus appears to consist of three dissimilar subunits.

Mode of degradation of precursor-specific ribonucleic acid fragments by Bacillus subtilis

A precursor of 5S ribosomal ribonucleic acid (rRNA) from Bacillus subtilis was cleaved by ribonuclease (RNase) M5 in cell-free extracts from B. subtilis to yield the mature 5S rRNA plus RNA fragments derived from both termini of the precursor. The released, mature 5S rRNA was stable in these extracts; however, as occurred in vivo, the precursor-specific fragments were rapidly and completely destroyed. Such destruction was not observed in the presence of partially purified RNase M5, so fragment scavenging was not effected by the maturation nuclease itself. The selective destruction of the precursor-specific fragments was shown to occur through a 3'-exonucleolytic process with the release of nucleoside 5'-monophosphates; the responsible activity therefore had the character of RNAse II. Consideration of the primary and probable secondary structures of the precursor-specific fragments and mature 5S rRNA suggested that involvement of 3' termini in tight secondary structure may confer protection against the scavenging activity.

Genetic analysis of mutations affecting ribonuclease II in Escherichia coli

Exonuclease activity in an Escherichia coli K12 mutant S296 is less than 1% of that in the wild type strain (Nikolaev et al., 1976). Another mutant N464 has thermolabile ribonuclease II (Castles and Singer, 1968; Kuwano et al., 1969). Genetic analysis of these mutants by Hfr conjugation and P1 transduction indicates that the structural gene (rnb) for ribonuclease II is located near the pyrF gene (28 min on the E. coli genetic map of Bachmann, Low and Taylor (1976)), and the most probable gene order is tyrT-trp-pyrF-rnb.

Processing by ribonuclease II of the tRNATyr precursor of Escherichia coli synthesized in vitro

The tRNATyr precursor molecule, synthesized from phi 80 psu3+ DNA (containing a single tRNA gene) by DNA-dependent RNA polymerase and q factor, was about 205 nucleotides long. The main product of its digestion with a ribonuclease tii preparation from Escherichia coli showed the same electrophoretic mobility as tRNAtyr precursor isolated in vivo and was found to be identical to it when analysed using fingerprint techniques. This intermediate precursor synthesized in vitro was converted further by processing with ribonuclease P into an RNA identical size to mature tRNATyr. It was concluded that the initiation of transcription of the tRNATyr gene in vitro occurs at the same site as that of transcription in vivo and a termination occurs at about 80 nucleotides beyond the CCA end of tRNATyr.