Isolation and characterization of large transfer ribonucleic acid precursors from Escherichia coli

Precise mapping and comparison of two evolutionarily related regions of the Escherichia coli K-12 chromosome. Evolution of valU and lysT from an ancestral tRNA operon

Two tRNA operons have been found near the gltX gene encoding the glutamyl-tRNA synthetase of Escherichia coli K-12. The alaW operon previously undetected from genetic data and containing two identical tRNA(GGCAla) genes is 800 base-pairs downstream from the gltX terminator and is transcribed from the same strand. The valU operon containing genes for three identical tRNA(UACVal) and one tRNA(UUULys) (the wild-type allele of supN), is adjacent to gltX and is transcribed from the opposite strand. Five open reading frames were also found in this region encoding putative polypeptides of 62, 105, 130, 167 and 294 amino acid residues. ORF294 is a new member of the lysR family of bacterial transcriptional activators. The possibility that this is the xapR gene is discussed. Comparison of the physical and linkage maps of the E. coli chromosome in the 52 minute region has permitted precise mapping of most of the 18 genes in this region with the order nupC-glk- less than (alaW beta-ala W alpha)-1 kb- less than gltX-0.3 kb-(valU alpha-valU beta-valU gamma-lysV = supN) greater than xapR-xapA- less than lig-1 kb-cysK greater than -0.4 kb-ptsH greater than -0.05 kb-pstI greater than -0.05 kb-crr greater than -cysM-cysA in the clockwise order (greater than and less than indicate the direction of transcription; kb, 10(3) bases). The last two genes of valU (52 min) and lysT (16.5 min) are arranged in a similar fashion and a highly conserved region has been found in both operons. This suggests that the valU and lysT operons probably arose by a duplication of an ancestral tRNA operon. This is the first example of what may be two different tRNA operons from the same organism evolving from an ancestral tRNA gene. Comparison of the 16 and 52 minute regions of the E. coli K-12 chromosome suggests that these two regions could share a common ancestor.

Genomic organization and physical mapping of the transfer RNA genes in Escherichia coli K12

By using a set of 476 ordered DNA clones (in lambda phage vector) that covers the entire chromosome of Escherichia coli K12, we have made an exhaustive survey of tRNA genes in the E. coli genome. Ultraviolet-irradiated bacteria were separately infected with each of the 476 clones and the RNA molecules produced upon infection were labeled with 32P. The labeled tRNAs were separated by gel electrophoresis and then characterized by fingerprinting analysis. Fifty-nine of the 476 clones produced tRNAs, including adjacent overlapping ones that share the same tRNA genes. The products of all the previously mapped tRNA genes (about 60, to date) were detected according to their expected positions, and 19 more tRNA genes were newly elucidated. These new tRNA genes were identified by sequencing the DNA from relevant regions of the clones; the DNA sequences were scanned for the stretches that could be folded into the familiar cloverleaf structure and the transcription units were deduced by predicting the promoters and terminators. The total complement of the tRNA genes in E. coli K12 was 78 for 45 tRNA (or 41 anticodon) species, distributed in 40 different transcription units throughout the chromosome. In addition, a gene for selenocysteine tRNA was detected by hybridization and mapped to a specific DNA segment. A comprehensive tRNA gene map of E. coli was constructed, including the selenocysteine tRNA gene. All the tRNA genes encode the 3' CCA, and in several cases the terminal 19 nucleotides (including the 3' CCA) of a tRNA gene is repeated several times. Finally, in the present study the sites for a long inversion (approx. 800 x 10(3) base-pairs, around the oriC region) in Kohara's library was determined to be within the 23 S-5 S regions in rrnD and rrnE, revealing the exchange of combinations of spacer and distal tRNA genes between these two ribosomal RNA operons.

The tRNA-tufB operon transcription termination and processing upstream from tufB

Two genes, tufA and tufB, located at 73 and 88 minutes of the Escherichia coli linkage map, code for the polypeptide chain elongation factor EF-Tu. tufB is transcribed with four upstream tRNA genes, thrU, tyrU, glyT and thrT, into a cotranscript of approximately 1800 nucleotides. Here we show that in vivo processing yields a 1320 nucleotide transcript of tufB. S1 nuclease fine mapping reveals that the processing site is located in the intergenic region at about 72 to 74 nucleotides upstream from the initiation codon of the tufB cistron. A deletion in the cloned tRNA-tufB operon, encompassing the 3' half of thrU, the complete tyrU, glyT, thrT genes and ten nucleotides of the intergenic region, causes a threefold increase of the rate of plasmid tufB transcription, a fourfold increase of plasmid-borne tufB RNA and a twofold increase of plasmid-borne EF-TuB. We conclude that the deletion has eliminated a transcription termination site probably located after the thrT gene. Termination at this site uncouples tRNA synthesis from tufB transcription.

Expression of the leuX gene in Escherichia coli. Regulation at transcription and tRNA processing steps

The leuX (supP) gene of Escherichia coli codes for a suppressor tRNA (tRNA(6Leu] that inserts leucine at the amber codon. Analysis of both in-vitro and in-vivo transcripts indicated that the gene is organized into a single gene operon, carrying its own promoter and rho-independent terminator, and its primary transcript accumulates in cells of wild-type E. coli with respect to tRNA processing. Systematic and quantitative measurements of both the unprocessed primary transcript and mature tRNA(Leu6) indicated that: (1) transcription of the leuX gene is under stringent control in vivo and is repressed in vitro by ppGpp; (2) transcription of the leuX gene is under growth rate-dependent control; but (3) the level of mature tRNA stays constant under various growth conditions. A model is proposed, which assumes that the enzyme catalyzing the first-step reaction in the leuX tRNA processing is limited, thereby keeping the level of mature tRNA(Leu6) at a constant level irrespective of changes in the level of the unprocessed primary transcript.

Sequence analysis of the glyW region in Escherichia coli

We have determined the DNA sequence of a 1-kilobase segment of the Escherichia coli chromosome. The segment contains glyW, a duplicate gene for tRNA3Gly, and its flanking regions. An insertion sequence, previously known to have occurred spontaneously within the sequenced fragment, was identified as IS1. Possible sites for initiation and termination of transcription were identified by comparing them with the sequences of model promoter regions and termination structures. The results suggest that the expression of glyW may depend upon the expression of the preceding gene, pgsA, by transcriptional or translational overlap, by cotranscription of these two genes, or both.

Identification of transfer RNA suppressors in Escherichia coli. III. Ochre suppressors of lysine tRNA

Transducing phages of lambda carrying suppressors, lysT (Su+ beta), supG and and supL, were isolated in vivo. Upon infection with each of these phages, the production of tRNALys and tRNAVal1 was markedly enhanced. Fingerprint analysis of these tRNAs revealed that they consisted of normal tRNALys, mutant tRNALys and tRNAVal1 in equimolar ratios. The mutant tRNALys carried a single-base alteration at the anticodon, from 5'-UUU-3' to 5'-UUA-3', which makes it an ochre suppressor. DNA sequence analysis of the entire transducing fragment (730 base-pairs) of lambda pSu+ beta revealed that three tRNA genes are tightly clustered within a transcription unit in the following order; i.e. promoter-(48 base-pairs)-wild-type tRNALys-(132 base-pairs)-tRNAVal1-(2 base-pairs)-Su+ beta tRNALys-. In wild-type bacteria there are two identical tRNALys genes in one operon. Although we have shown that in Su+ beta it is the distal tRNALys that has been mutated to the ochre suppressor by a single base change at the anticodon (U36 to A36), we have not determined which of the two genes bears the supG or the supL mutation. The sequences following both tRNALys genes are highly homologous: both are about 100 base-pairs long and both terminate with an 18 base-pair sequence homologous to the last 18 bases of each tRNA. The sequences of tRNALys and tRNAVal1 are also very similar. Thus, including the 3'-portions of these tRNA genes, the 18 base-pair sequence is more or less periodically repeated five times in the DNA sequence.

Structure of an Escherichia coli tRNA operon containing linked genes for arginine, histidine, leucine, and proline tRNAs

A plasmid containing a gene for the most abundant Escherichia coli leucine isoacceptor tRNA, tRNALeu1 (anticodon CAG) was isolated from the Clarke-Carbon bank of cloned E. coli DNA. The clone contains a 12.3-kilobase DNA insert which was mapped by F' DNA hybridization analysis to the region 82 to 89 min on the chromosome. The cloned tDNALeu corresponds to the minor of two chromosomal regions containing different amounts of DNA complementary to tRNALeuCAG . Sequencing of the tDNA region revealed it to contain a multimeric transcription unit consisting of four different tRNA genes. The genes are in the arrangement 5'-leader- tRNAArgCCG -57 base pairs- tRNAHisGUG -20 base pairs- tRNALeuCAG -42 base pairs- tRNAProUGG -3'. Coordinate expression of the component tRNAs in vivo and the absence of intercistronic promoters indicated that all four tDNAs reside in the same operon. The tDNA sequence is bounded by a promoter element showing good agreement with the procaryotic consensus sequence and a GC-rich stem-loop element that corresponds to a rho-independent terminator. The promoter region contains a GC-rich sequence that agrees with a suggested consensus stringency control element and two domains possessing dyad symmetry which flank the Pribnow box and include the putative stringency control region.

Molecular mapping of glyW, a duplicate gene for tRNA3Gly of Escherichia coli

By the use of [5'-32P]tRNA3Gly from Escherichia coli as a hybridization probe, glyW was located on cloned fragments of the uvrC pgsA region of the bacterial chromosome. After determination of the sites of action of several restriction enzymes, glyW was found to be within approximately 300 base pairs of pgsA. The order of genes in this region is uvrC, pgsA, glyW, flaI. Comparison of the order of determined restriction sites with the sites predicted from the nucleotide sequence of tRNA3Gly indicates that the direction of transcription of glyW is counterclockwise on the circular E. coli map.

Characterization of a cold-sensitive hisW mutant of Salmonella typhimurium

Previous studies of hisW mutants of Salmonella typhimurium have led to the suggestion that such strains are defective in tRNA maturation. (J. E. Brenchley and J. Ingraham, J. Bacteriol. 114:528-536, 1973). In this study, we report that one hisW strain is defective in the accumulation of all stable RNA species. Polyacrylamide gel electrophoresis of radiolabeled RNA indicated tha at the nonpermissive temperature (20 degrees C) all stable RNa species in the cold-sensitive hisW3333 mutant were synthesized and rapidly degraded. We propose that the cold sensitivity of this strain is caused by such a restriction in stable RNA accumulation at low temperature. In vitro and in vivo studies demonstrated that the RNA degraded in this strain was synthesized de novo and was not preexisting RNA. Furthermore, physiological and genetic recovery from the cold-sensitive hisW phenotype resulted in relatively normal RNA synthesis and accumulation. Thus, the RNA alterations observed in this strain were not explained by defects in a tRNA modification enzyme. Rather, these findings suggest the existence of defective RNA processing and that a control mechanism for the overall synthesis or accumulation of stable RNA species is altered in the hisW3333 mutant.

Dual function transcripts specifying tRNA and mRNA

A cluster of four tRNA genes in Escherichia coli is co-transcribed with an adjacent gene encoding elongation factor Tu. The resultant transcript that specifies both structural (tRNA) and informational (mRNA) RNA may not be an uncommon occurrence and has interesting regulatory implications.

Location of the tufB promoter of E. coli: cotranscription of tufB with four transfer RNA genes

Previous nucleotide sequence studies have demonstrated that the structural genes for four transfer RNA species (thrU, tyrU, glyT and thrT) are positioned close to one another and near tufB, one of the structural genes for elongation factor Tu. We have carried out experiments to determine the position of the tufB promoter and thus to infer whether these five genes are in a single transcription unit. tufB cloned on plasmids was fused to Tc (in operon fusion) or to lacZ (in gene fusion). These plasmids were then subjected to in vitro deletion to locate the promoter responsible for tufB transcription. In addition, the ability of wild-type tufB to complement kirromycin resistance was determined with deletion plasmids. The results indicate that the major promoter for tufB lies upstream from the four transfer RNA genes, and that there might be at least one weak internal promotor, possibly adjacent to tufB. Assuming that the four tRNA genes that lie between the major promoter and tufB are also transcribed from that promoter, we suggest that all five genes lie in a single transcription unit (thrUp-thrU-tyrU-glyT-thrT-tufB-tufBt) whose primary transcript is thus both a transfer RNA precursor and messenger RNA.

Transfer RNA precursors are accumulated in Escherichia coli in the absence of RNase E

A temperature-sensitive Escherichia coli mutant, which contains a heat-labile RNase E, fails to produce 5-S rRNA at a non-permissive temperature. It accumulates a number of RNA molecules in the 4-12-S range. One of these molecules, a 9-S RNA, is a precursor to 5-S rRNA [Ghora, B. K. and Apirion, D. (1978) Cell, 15, 1055-1056]. These molecules were purified and processed in a cell-free system. Some of these RNA molecules, after processing, give rise to products the size of transfer RNA, but not to 5-S-rRNA. Further characterization of the processed products of one such precursor molecule shows that it contains tRNA1Leu and tRNA1His. RNase E is necessary but not sufficient for the processing of this molecule to mature tRNAs in vitro. The accumulation of such tRNA precursors in an RNase E mutant cell and the obligatory participation of RNase E in its processing indicate that RNase E functions in the maturation of transfer RNAs as well as of 5-S rRNA.

Organization of transfer ribonucleic acid genes in the Escherichia coli chromosome

The arrangement of transfer ribonucleic acid (RNA) genes in the chromosome of Escherichia coli K-12 (C600) was examined with the techniques of restriction endonuclease digestion and Southern blotting. The number and size of restriction fragments containing transfer or ribosomal RNA sequences or both were estimated by a variety of restriction endonucleases, including EcoRI, BglI, SmaI, SalI, BamHI, and PstI. EcoRI liberated a minimum of 27 fragments which hybridized to transfer RNA and 16 which hybridized to ribosomal RNA. Enzymes which did not cut within the ribosomal RNA operons (PstI and BamHI) liberated 16 and 13 fragments, respectively, which hybridized to transfer RNA. Five PstI and six BamHi fragments also hybridized to ribosomal RNA, suggesting that there may be at least 11 chromosomal locations distinct from ribosomal RNA operons which encode transfer RNA genes. In addition, our data indicated that several transfer RNA genes may be very close to the 5' proximal ends of certain ribosomal RNA operons and close to the 3' distal ends of all seven ribosomal RNA operons. Similar studies have been carried out with 22 purified species of transfer RNA, and we report here the number and size of EcoRI restriction fragments which hybridize to these transfer RNA species.

A double base change in alternate base pairs induced by ultraviolet irradiation in a glycine transfer RNA gene

The glyUsuAGA mutation affects Escherichia coli tRNA Gl y GGG, changing it to an AGA missense suppressor tRNA. Sequence studies have shown that the mutation involves a double base subsitution at the first and third positions of the tRNA anticodon, the result being a change in the anticodon from CCC to UCU. A system has been developed to facilitate the detection of this novel mutation, and we have shown that ultraviolet irradiation and N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) are effective in causing the double base change. A single observation of the mutation occurring spontaneously has been made also. The frequency of MNNG-induced glyUsuAGA mutations is compatible with their being caused by two separate mutagenic events. The frequency of UV-induced glyAGA mutations, however, strongly suggests that the occurrence of one base substitution strongly enhances the chance of finding the second substitution at the alternate position. In addition to the double change in the anticodon, the glyUsuAGA tRNA differs from tRNA Gl y GGG in that it bears a modification of the A adjacent to the 3' position of the anticodon. Most likely, this modified base is N-[9-(beta-D-ribofuranosyl)-purin-6-ylcarbamoyl] threonine.

Characterization of 10S RNA: a new stable rna molecule from Escherichia coli

When cells of Escherichia coli are labeled with 32Pi for long periods of time and the cell content is subjected to electrophoresis in polyacrylamide gels, an RNA band appears which is about 10S in size. This band seems to contain three conformers. After treatment with formamide only a single band appears in this region of the gel, which contains 550 nucleotides as determined from its mobility. The complexity of the fingerprint of this material, after digestion with T1-RNase, is in agreement with the size as determined by the mobility, this confirming that indeed it is a single molecule. Composition of the T1-oligonucleotides was determined by digesting the T1-generated oligonucleotides with pancreatic RNase and T2-RNase. The quantitative and qualitative analysis of these digestions suggest that 10S RNA contains 609 nucleotides. The molecule contains, besides the four regular bases, one copy per molecule of the modified base pseudouridine. 10S RNA cannot be processed by cell extracts to tRNA-sized molecules and does not bind significantly to ribosomes, hence it is unlikely to be a tRNA precursor or an mRNA.

Structure and organization of the two tRNATyr gene clusters on the E. coli chromosome

The structure and organization of the gene clusters coding for the two tyrosine-accepting tRNA species (tRNA1Tyr and tRNA2Tyr) on the E. coli chromosome have been determined. The mature structural sequences of the two tRNATyr genes, located on opposite sides of the E. coli chromosome, differ by only 2 bp, but sequences surrounding these portions of the genes are very different. The genes coding for tRNA1Tyr (tyrT) comprise two mature structural sequences separated by a 200 bp "intergenic spacer." It is known that in transducing phage, the region adjoining the CCA end of the second mature structural sequence comprises a 178 bp repeated sequence which contains an in vitro, rho-dependent transcriptional termination site. We find that these potentially genetically unstable repeated sequences are present in the E. coli chromosome with the same organization as that determined from transducing phage analyses. The gene that codes for tRNA2Tyr (tyrU) is present in a single copy and is tightly clustered with three other tRNA genes. One of these genes (to be called thrU) encodes a previously undescribed tRNA (to be called tRNA4Thr). The organization of this cluster on the E. coli chromosome is tRNA4Thr--8 bp--tRNA2Tyr--115 bp--tRNA2Gly--6 bp--tRNA3Thr. The importance of correlating structural analyses derived from specialized transducing phage with those determined for the chromosome itself is demonstrated by results which show that out of four independently isolated tRNATyr transducing phage, two carrying the tRNA1Tyr genes [phi80psu3+,- (Cambridge) and phi80sus2psu3+ (Kyoto)] and two carrying the tRNA2Tyr gene (lambdarifd 18 and lambdah80dglyTsu+36), only the first phage from each group has the same gene organization as that found in the E. coli chromosome.

Organization of genes for transcription and translation in the rif region of the Escherichia coli chromosome

The lambdarifd18 transducing phage is known to carry several genes for components of transcriptional and translational machineries; these genes are clustered in the rif region at 88 min on the Escherichia coli genetic map. They include a set of genes for rRNA's (rrnB), a gene for spacer tRNA, tRNA2Glu (tgtB), one of the two genes for EF-Tu (tufB), genes for four ribosomal proteins (rplK, A, J, and L), genes for the beta and beta' subunits of RNA polymerase (rpoB and rpoC), and genes for three tRNA's (tyrU, gluT, and thrT). An additional tRNA gene (subsequently identified as thrU by Landy and his co-workers) and a gene for a protein (protein U) with unknown functions were found to be carried by lambdarif d18. We analyzed the organization of these genes by using various deletion and hybrid phages derived from lambdarif d18 and lambdarif d12, a phage related to lambdarif d18. The expression of various genes was examined in UV-irradiated cells infected with these transducing phages. Two main conclusions were obtained. First, the four tRNA genes are not cotranscribed with the genes in rrnB, even though these tRNA genes are located close to the distal end of rrnB. Second, the four ribosomal protein genes are organized into two separate transcriptional units; rplK and A are in one unit and rplJ and L are in the second unit. The first group of genes was shown to have a promoter separate from that for tufB or protein U. The second group of genes shares the promoter with rpoB and C, as described in a separate paper (M. Yamamoto and M. Nomura, Proc. Natl. Acad. Sci. U.S.A., 75:3891--3895). These and other results described in this paper show that the genes are organized in the following order: promoter, genes in rrnB; promoter, thrU, tyrU, (promoter?) glyT, thrT; (promoter?) tufB; promoter, a gene for protein U; promoter, rplK, rplA; promoter, rplJ, rplL, rpoB, rpoC.

Processing of rRNA by RNAase P: spacer tRNAs are linked to 16S rRNA in an RNAase P RNAase III mutant strain of E. coli

To determine which enzymes are responsible for the processing cleavages of ribosomal RNA transcripts in Escherichia coli, we constructed a mutant strain lacking RNAase III and containing a thermolabile RNAase P. At the nonpermissive temperature, this strain accumulates a novel "19S" RNA species which contains 17S precursor rRNA sequences covalently linked to tRNA sequences transcribed from the ribosomal RNA spacer region between the 16S and the 23S rRNA cistrons. In vitro treatment of 19S RNA with cell extracts releases tRNA2Glu and other tRNA species. These "spacer" tRNA sequences are apparently not contained with the 18S RNA species found in an RNAase III- RNAase P+ cell. RNAase P-deficient extracts are incapable of cleaving space tRNA from 19S RNA, indicating that RNAase P is required for the release of spacer tRNAs from rRNA transcripts of E. coli cells.

Specific ribonucleases involved in processing of tRNA precursors of Escherichia coli. Partial purification and some properties

Ribonucleases O and Q, the two putative nucleolytic activities which we detected previously in the crude extract from a thermosensitive ribonuclease P mutant (TS241) of Escherichia coli and which were shown to function in the processing of tRNA precursors in vitro, were partially purified from the 1000000 x g supernatant fraction of E. coli Q13. In the course of purification of these enzymes, the total RNAs synthesized in the thermosensitive mutant at the restrictive temperature were used as the substrates and the activities were identified from disappearance or alteration of specific tRNA precursor molecules in polyacrylamide gel electrophoresis. The purified ribonuclease O preparation cleaved specifically the multimeric tRNA precursors at the spacer regions. The purified ribonuclease Q preparation removed, in accordance with the definition of this enzyme, extra nucleotides from the 3'-terminal ends of monomeric tRNA precursors. Some properties of these two nucleases were investigated. In addition to these nucleases, another exonuclease (tentatively designated ribonuclease Y) and ribonuclease P, a well-characterized endonuclease, were also purified. The sequential mode of the processing of tRNA precursors, originally observed in the cleavage reactions with the crude extracts in vitro, was supported by studies with the purified enzyme preparations.

Small stable RNAs from Escherichia coli: evidence for the existence of new molecules and for a new ribonucleoprotein particle containing 6S RNA

Small stable RNA molecules of Escherichia coli other than 5S (rRNA) and 4S (tRNA) were studied. Two of the molecules corresponded to 4.5S and 6S RNA, which have been reported previously. The third stable RNA molecule, 10S RNA, has not been described before. RNA labeled with (32)P(i) or [(14)C]uracil for a relatively long time, when separated in 5%/12% tandem polyacrylamide gels, displayed three bands corresponding to 10S, 6S, and 4.5S RNA in addition to rRNA and tRNA bands. These RNAs were stable in pulse-chase-labeling experiments. The amount of these RNAs was small, comprising only 0.2 to 0.5% of the total (32)P incorporation. However, this amount represented a large number of molecules; for 6S and 4.5S, it was about 1,000/DNA molecule. These three RNAs were found in the postribosomal supernatant fraction. None of them was found in purified nucleoid fractions in which the tightly coiled DNA molecules were contained. Of these three RNAs, 6S RNA was unique in that it seemed to exist in a ribonucleoprotein particle. All these RNAs, as well as tRNA, were very stable in the cell under various physiological conditions. 5S RNA was less stable. On the other hand, purified 6S RNA was more susceptible than tRNA to cell nucleases when incubated with cell extracts, suggesting that, being in a particle, it is protected from cell nucleases.

Fine structure physical mapping of 4S RNA genes on mitochondrial DNA of Saccharomyces cerevisiae

We have localized the genes for mitochondrial 4S RNA on the physical map of the mtDNA of several Saccharomyces cerevisiae strains by hybridization of iodinated 4S RNA to the restriction fragments obtained with endonucleases HindII + III, EcoRI and HapII. The data indicate that 5-8 of the 4S RNA genes are dispersed over a large area of the genome whereas the rest (about 18 genes) is located within an area of about 9000 bp in length (about 18 genes) is located within an area of about 9000 bp in length (about 12% of the genome) between the markers for chloramphenicol and paromomycin resistance (RIB 1 and PAR 1 loci). Within this region a cluster is present of 5 genes on a DNA fragment of 460 bp.

Spacer transfer RNAs in ribosomal RNA transcripts of E. coli: processing of 30S ribosomal RNA in vitro

At least three different transfer RNAs are produced by in vitro processing of 30S ribosomal RNA which accumulates in RNAase III- strains of E. coli. Two of these tRNAs, tRNAGlu2 and tRNAIle1, have previously been shown to be "spacer tRNAs"--that is, genes for their synthesis are located in rRNA transcription units between the cistrons for 16S and 23S rRNAs (Lund et al., 1976). The third tRNA whose sequences are contained in 30S rRNA is tRNAAla1B. In addition to the tRNAs, 5S rRNA and several other 4S fragments are produced. Some of these 4S fragments may represent additional spacer tRNAs. One fragment, about 70 nucleotides long, arises from the 5' end of the 17S precursor of 16S rRNA. Four or five other tRNAs are hydrogen-bonded to 30S rRNA as we prepare it; one or more of these tRNAs may also be a spacer tRNA. The enzymes that process tRNAs out of 30S rRNA are associated with ribosomes, but can be removed from them by washing in 0.2 M NH4Cl; the enzymes required for 5S rRNA processing remain bound to the 0.2 M NH4Cl-washed ribosomes. Treatment of 30S rRNA with purified RNAase III produces 6-8S fragments which contain the sequences of tRNAGlu2, tRNAAla1B and 5S rRNA.