A comparison of transcriptional linkage of tRNA cistrons in yeast and E. coli by the ultraviolet light technique

tRNA genes and retroelements in the yeast genome

A survey of tRNA genes and retroelements (Ty) in the genome of the yeast Saccharomyces cerevisiae is presented. Aspects of genomic organization and evolution of these genetic entities and their interplay are discussed. Attention is also given to the relationship between tRNA gene multiplicity and codon selection in yeast and the role of Ty elements.

Analysis of yeast chromosomal regions carrying members of the glutamate tRNA gene family: various transposable elements are associated with them

We carried out an analysis on the genomic organisation of the tRNA(Glu) family in S. cerevisiae; eight clones were characterized by restriction mapping, hybridization and sequencing. These data taken together with our earlier findings show that the individual tRNA(Glu3) copies are identical only in their structural part but embedded in entirely different genomic environments. All of the tRNA genes identified here are flanked by elements such as Ty, delta, sigma, and tau. In some cases, sequences from different elements form complex patterns indicating a sophisticated history of these chromosomal regions. A novel observation is that Ty and delta in the regions analyzed are exclusively associated with the tRNA genes. The observed patterns imply that the tRNA genes mark regions of multiple transposition and subsequent excision events, but that these have occurred after the individual tRNA gene copies had been fixed in their present locations. Transcription experiments by the use of micro-injection into Xenopus oocytes suggest that the elements flanking the tRNA genes exert a modulating effect on their expression.

One member of the tRNA(Glu) gene family in yeast codes for a minor GAGtRNA(Glu) species and is associated with several short transposable elements

During characterization of the whole tRNA-(Glu) family from the yeast, Saccharomyces cerevisiae, we isolated one cosmid clone bearing a tRNA(Glu) gene copy that is deviant from the major tRNA(Glu3) gene members in only five positions. This divergent tRNA-(Glu) is a minor species and is represented by a single gene copy. One of the nucleotide exchanges concerns the anticodon which is modified from T-T-C in the tRNA(Glu3) gene to C-T-C which implies that this tRNA serves the codon triplet G-A-G. Two other minor yeast tRNA species have been reported which appear to be particularly designed for the translation of those codons that have a G in its third (Wobble) position. The low abundance of such minor tRNA species correlates positively to the low occurrence of most of the N-N-G codons in yeast. Furthermore, the GAGtRNA-(Glu) locus represents another case of the general phenomenon in which the majority of the tRNA genes in yeast are associated with one or several transposable elements forming complex patterns. In this particular case, divergent segments of delta and tau are present in the 5' flanking region of the tRNA gene and arranged in a novel configuration. The sequence data lend support to the view that tau is not an evolutionary young element as was earlier anticipated.

Dimeric tRNA precursors in yeast

Two DNA fragments, each containing tRNA(Arg)3 and a tRNA(Asp) gene in close conjunction, have been isolated from different genomic regions of Saccharomyces cerevisiae. Nucleotide Nucleotide sequence analysis of the gene regions revealed that in both fragments the tRNA(Arg)3 coding region is located 5'-proximal to the tRNA(Asp) coding region. They are separated by an identical spacer of 10 nucleotides. Although the 5'-flanking sequences are different in the two plasmids, some similarities are observed. To test the mode of expression of this gene configuration, we transcribed the DNA fragments in a Xenopus oocyte nuclear extract. Specific transcription of the yeast tRNA genes took place in an RNA precursor which comprised both tRNA species. We report here that the precursor RNA was processed to the mature-sized tRNA molecules, indicating the presence of an enzyme activity in the Xenopus nucleus capable of cutting a dimeric tRNA precursor. This is the first observation of a eukaryotic dimeric tRNA precursor.

Dimeric transfer RNA precursors in S. pombe

Sequence analysis of a Schizosaccharomyces pombe DNA fragment revealed two tRNA coding regions separated by a seven nucleotide spacer. the 5'-proximal tRNA gene encodes a tRNAUCGSer sequence, which is interrupted by a 16 nucleotide intron at the 3' side of the base adjacent to the anticodon. The second tRNA gene encodes an initiator tRNAMet sequence. This DNA fragment, cloned into pBR322, was used as template for in vitro transcription in a nuclear extract of Xenopus oocytes. The tRNA genes were transcribed into one RNA precursor which contained both tRNA sequences. The primary transcription product initiates with pppG, contains a 9 nucleotide leader sequence, a 16 nucleotide intron, a 7 nucleotide spacer between the two tRNA molecules and an 8--9 nucleotide trailer sequence. RNA initiation was only observed upstream of the 5'-proximal tRNASer. We used RNA analysis to establish a sequence of the enzymatic steps of tRNA maturation in the nuclear extract. The first step in processing the dimeric precursor is an endonuclease cleavage which generates the mature 5' end of the tRNAMet. Further steps include the removal of the flanking sequences and addition of the CCAOH 3' terminus. The last step is the splicing of the tRNASer precursor to remove the intervening sequence.

Fractionation and identification of spinach chloroplast transfer RNAs and mapping of their genes on the restriction map of chloroplast DNA

Spinach chloroplast 4S RNAs has been separated by two-dimensional polyacrylamide gel electrophoresis into about 35 species. After extraction from the gel, 27 of these RNA species were identified by aminoacylation as tRNAs specific for 16 amino acids. Individual tRNAs were labeled in vitro with 125I and hybridized to DNA fragments obtained by digestion of spinach chloroplast DNA with KpnI, PstI, SalI and XmaI restriction endonucleases. A minimum of 21 genes corresponding to tRNAs for 14 different amino acids have been localized on the restriction endonuclease cleavage site map of the DNA molecule. Of these, 15 genes corresponding to tRNAs for 12 amino acids are located in the larger of the two single-copy regions which separate the two inverted copies of the repeat region. Each copy of this repeat region contains a set of genes for the ribosomal RNAs and a gene for tRNA2Ile in the "spacer" sequence between the 16S and 23S ribosomal RNAs. The genes for tRNA1Ile, tRNA2Leu and tRNA3Leu also map in the repeat region, but outside the ribosomal DNA unit. At present, two more chloroplast tRNAs (for Pro and Lys) have been identified, but not mapped, while 4 unidentified 4S RNAs have been mapped in the large single-copy region of the DNA molecule. Evidence is presented that isoaccepting tRNA species can be transcripts from different loci.

Studies on tRNA adaptation, tRNA turnover, precursor tRNA and tRNA gene distribution in Bombyx mori by using two-dimensional polyacrylamide gel electrophoresis

Eighteen out of twenty amino acids have been used for identifying tRNAs from the silkworm Bombyx mori L. fractionated on two-dimensional polyacrylamide gel electrophoresis. 43 spots out of 53 have been identified. This mapping confirms previous results and brings new answers to some questions on the regulation of tRNA biosynthesis. 1. In addition to quantitative adaptation of tRNAs to the composition of silk proteins (fibroin from the posterior silk gland, sericin from the middle part) and of iso-tRNAs from posterior silk gland to the major codons of fibroin mRNA, we also observe adaptation of tRNA from various tissues to the average amino acid content of proteins from fat body, gut, gonads and carcass of the silkworm. 2. In the silk gland, turnover rates of several tRNA species are similar. The selective accumulation of tRNAs needed for decoding fibroin and sericin mRNAs which takes place during the Vth larval instar, cannot be explained by the occurrence of a preferential degradation of some tRNA species. 3. Under given conditions for incubating silk glands, it is possible to obtain an accumulation of precursor tRNA species, which are enriched in pre-tRNAAla and pre-tRNAGly in the posterior silk gland and pre-tRNASer in the middle part. 4. The distribution of tRNA genes is not random. tRNA genes for glycine, alanine and serine are prominent. Selective transcription of batteries of iso-tRNA genes could explain our data.

Structure and processing of yeast precursor tRNAs containing intervening sequences

We have isolated a precursor of yeast tRNATyr and shown that it contains an intervening sequence identical to that found in the gene for tRNATyr. The conformation of pre-tRNATyr is similar to that of mature tRNATyr except for the anticodon loop. The loop is sensitive to endonucleolytic cleavage by S1 nuclease near to the ends of the intervening sequence. This pre-tRNA is functionally inactive as it cannot be aminoacylated and the anticodon is not accessible for hydrogen bonding. A crude nuclear extract from yeast contains an excision-ligase activity which will process pre-tRNATyr into mature tRNATyr.

Structure of yeast phenylalanine-tRNA genes: an intervening DNA segment within the region coding for the tRNA

Sixteen bacterial clones containing sequences complementary to yeast PhetRNA were isolated from a collection of hybrid plasmids containing BamHI restriction endonuclease-generated yeast DNA fragments inserted in the plasmid vector pBR315. Ten of these clones contained hybrid plasmids with distinct BamHI fragments. The sequence of the Phe-tRNA structural genes and adjacent regions of three of these clones is reported here. In the region flanking the tRNA gene, the sequence of two of the cloned DNAs is similar; the sequence of the third varies considerably. All three of the tRNA genes are bordered by A,T-rich regions. In particular, near the region coding for the 3' end of the tRNA there is a long sequence of As in the coding strand. This is reminiscent of the region of termination of transcription of the yeast 5S rRNA gene. The sequences coding for the Phe-tRNA contain an additional segment of 18 or 19 base pairs (depending upon the clone) not predicted by the yeast Phe-tRNA sequence. These intervening segments are nearly identical in the three clones and are located within the structural gene, two base pairs from the nucleotides coding for the tRNA anticodon.