Biochemical and physical characterization of an unmodified yeast phenylalanine transfer RNA transcribed in vitro

A recombinant plasmid was constructed with six synthetic DNA oligomers such that the DNA sequence corresponding to yeast tRNA(Phe) is flanked by a T7 promoter and a BstNI restriction site. Runoff transcription of the BstNI-digested plasmid with T7 RNA polymerase gives an unmodified tRNA of the expected sequence having correct 5' and 3' termini. This tRNA(Phe) transcript can be specifically aminoacylated by yeast phenylalanyl-tRNA synthetase and has a Km only 4-fold higher than that of the native yeast tRNA(Phe). The Km is independent of Mg2+ concentration, whereas the Vmax is very dependent on Mg2+ concentration. Comparison of the melting profiles of the native and the unmodified tRNA(Phe) at different Mg2+ concentrations suggests that the unmodified tRNA(Phe) has a less stable tertiary structure. Using one additional DNA oligomer, a mutant plasmid was constructed having a guanosine to thymidine change at position 20 in the tRNA gene. A decrease in Vmax/Km by a factor of 14 for aminoacylation of the mutant tRNA(Phe) transcript is observed.

Mutants affecting tRNA(Phe) from Escherichia coli. Studies of the suppression of thermosensitive phenylalanyl-tRNA synthetase

Four mutants of pheV, a gene coding for tRNA(Phe) in Escherichia coli, share the characteristic that when carried in the plasmid pBR322, they lose the capacity of wild-type pheV to complement the thermosensitive defect in a mutant of phenylalanyl-tRNA synthetase. One of these mutants, leading to the change C2----U2 in tRNA(Phe), is expressed about 10-fold lower in transformed cells than wild-type pheV. This mutant, unlike the remaining three (G15----A15, G44----A44, m7G46----A46), can recover the capacity to complement thermosensitivity when carried in a plasmid of higher copy number. The other three mutants, even when expressed at a similar level, remain unable to complement thermosensitivity. A study of charging kinetics suggests that the loss of complementation associated with these mutants is due to an altered interaction with phenylalanyl-tRNA synthetase. The mutant gene pheV (U2), when carried in pBR322, can also recover the capacity to complement thermosensitivity through a second-site mutation outside the tRNA structural gene, in the discriminator region. This mutation, C(-6)----T(-6), restores expression of the mutant U2 to about the level of wild-type tRNA(Phe).

A synthetic substrate for tRNA splicing

A novel method for the synthesis of precursor tRNA as substrate for in vitro splicing is reported. A construct consisting of the Saccharomyces cerevisiae pre-tRNAPhe gene under the control of a bacteriophage T7 promoter was assembled from a set of synthetic oligonucleotides and cloned into an M13 vector. By the use of T7 RNA polymerase, BstNI-runoff transcripts were produced. The resulting pre-tRNA was shown to possess mature termini and was accurately spliced by highly purified yeast tRNA-splicing endonuclease and ligase. Using this synthetic pre-tRNA, the kinetic parameters of the tRNA-splicing endonuclease were also determined. Use of this system provides several advantages for the study of tRNA-splicing mechanisms. Mutant tRNA precursors can be readily synthesized. It is also possible to synthesize large quantities of pre-tRNA for structural studies.

Expression of a set of synthetic suppressor tRNA(Phe) genes in Saccharomyces cerevisiae

Synthetic ochre and amber tRNA suppressor genes derived from the yeast tRNA(PheGAA) sequence have been constructed. They were efficiently transcribed in vitro and expressed in vivo via a synthetic expression cassette. tRNA(PheUUA) and tRNA(PheUUA) delta IVS (IVS = intervening sequence) are relatively inefficient ochre suppressors. They are toxic to the cell when expressed on a multicopy plasmid, and they do not suppress at all when present as single copies. The intron does not seem to have any effect on suppression. In contrast, the amber suppressor tRNA(PheCUA) delta IVS is efficient when expressed from a single-copy plasmid, while its efficiency is reduced on a multicopy vector.

The yeast tRNA(Phe) gene family: structures and transcriptional activities reveal member differences not explained by intragenic promoters

Several cloned members of the yeast tRNA(Phe) gene family were transcribed in vitro using a HeLa extract and a yeast extract. The optimum DNA concentration was determined and kinetic experiments were performed for each clone to compare transcription levels. Both extract systems were able to splice the intervening sequence, but only the yeast extract produced the mature product. Some genes were not transcribed with the homologous system while they were transcribed with the HeLa extract, suggesting a control mechanism that is not operating in the heterologous system. Competition experiments demonstrated that the intragenic promoters of the inactive genes were able to bind transcription factor(s), but not as efficiently as active genes. This binding was not so strong when using linear DNA and was dependent on the presence of the 3' intragenic control region. DNA sequencing and computer analysis indicated the presence of short conserved sequences upstream from the genes. These sequences, which are not related to the intragenic promoters, are direct repeats of part of the 3' coding region in those genes that are transcribed in the homologous system. The relevance of these sequences on homologous transcription in vitro remains to be established.

Homology in the region containing a tRNA(Trp) gene and a (complete or partial) tRNA(Pro) gene in wheat mitochondrial and chloroplast genomes

We have used bean mitochondrial (mt) and chloroplast (cp) tRNA(Trp) as probes to locate the corresponding genes on the mt and cp genomes of wheat and we have determined the nucleotide sequences of the wheat mt and cp tRNA(Trp) genes and of the flanking regions. Sequence comparisons show that the wheat mt and cp tRNA(Trp) genes are 97% homologous. On the wheat cp DNA, a tRNA(UGGPro) gene was found 139 bp upstream of the cp tRNA(Trp) gene. On the wheat mt DNA, a sequence of 23 nucleotides completely homologous with the 3' end of this cp tRNA(Pro) gene was found 136 bp upstream of the mt tRNA(Trp) gene, but there is only 38% homology between cp and mt wheat genomes in the intergenic regions. The overall organization of this region in the chloroplast genome (a tRNA(Trp) gene separated by about 140 bp from a tRNA(Pro) gene) is also found in the mitochondrial genome, suggesting that this mitochondrial fragment might have originated from a chloroplast DNA insertion. A comparison of the genes and of the intergenic regions located between the tRNA(Trp) gene and the tRNA(Pro) (or partial tRNA(Pro)) gene shows that there is an almost complete conservation of these sequences in the mitochondrial DNA of wheat and maize, whereas wheat mt and cp intergenic regions show more sequence divergence. Wheat mt tRNA(Trp) gene is encoded by the main mt genome (accounted for by the master chromosome) but, in the case of maize mitochondria, this gene was found to be encoded by the 2.3 kb linear plasmid, indicating that this plasmid is not dispensable in maize mitochondria.

Analysis of a human gene cluster coding for tRNA(GAAPhe) and tRNA(UUULys)

A 13.8-kb fragment of human DNA isolated from a human lambda Charon-4A DNA library was found to contain four human tRNA genes. Nucleotide sequence analysis of approx. 3.7 kb of this segment of human DNA identified two lysine tRNA(UUU) genes identical in coding sequence to a previously reported human lysine tRNA gene [Roy et al., Nucl. Acids Res. 10 (1982) 7313-7322]. The other two tRNA genes were phenylalanine tRNA(GAA) genes, the first to be isolated from a mammalian source. These phenylalanine tRNA(GAA) genes were identical in sequence with the exception of a G/A polymorphism at coordinate 57. None of these tRNA genes contains introns. The tRNA(UUULys) and tRNA(GAAPhe) genes are organized in alternating order and are irregularly spaced, by intergenic regions of approx. 1.0, 2.6 and 5.0 kb, and randomly oriented. There was no evidence to indicate that any of these genes arose by gene duplication, since flanking sequence homology was limited to the putative RNA polymerase III termination signals in the 3'-flanking regions. A mature tRNA-sized product was identified following the transcription of each tRNA gene in a homologous in vitro transcription system. Interestingly, different levels of transcriptional activity of the three identical lysine tRNA genes were observed, suggesting modulation of tDNA expression by extragenic sequences. In addition, a minimum of eight regions of homology to Alu-type repetitive elements were detected in this human DNA fragment, one of which was located 53 bp upstream from a tRNA(GAAPhe) gene.

Phenylalanine and tyrosine transfer RNAs encoded by Tetrahymena pyriformis mitochondrial DNA: primary sequence, post-transcriptional modifications, and gene localization

We have isolated Phe and Tyr tRNAs from Tetrahymena pyriformis mitochondria and have determined that these are "native" species, encoded by the mtDNA. A single gene for the tRNA(Phe) has been positioned 12-14 kbp from the left end of the linear Tetrahymena mtDNA, while duplicate tRNA(Tyr) genes have been localized within the inverted terminal repeats of this genome. Primary sequence analysis demonstrates that the tRNA(Tyr) has all of the characteristic primary and secondary structural features of a normal tRNA; however, the tRNA(Phe) displays several atypical features, including (i) replacement of the usual T psi sequence by UC, (ii) a U.U pair in the T psi C stem, and (iii) an extra 5'-nucleotide (U).

Correlation between three-dimensional structure and chemical reactivity of transfer RNA

The bases of yeast tRNA(Phe) which react with carbodiimide and methoxyamine have been determined and this information has been combined with chemical modification studies of other workers to produce a composite picture of base accessibility in this tRNA. The results are compared with the three-dimensional structure which we have recently determined. The bases which react chemically lie in exposed positions in the three-dimensional model and those which do not are either in the double helical stem regions or else are involved in maintaining the tertiary structure through pairing or stacking interactions.