Implication of 5'-flanking sequence elements in expression of a plant tRNA(Leu) gene

A comparison of 5'-flanking sequences from 68 different nuclear plant tRNA genes was analyzed to find consensus sequences. Three conserved features stood out, all of which are present in the tRNA(Leu) gene used in this study: (1) a high proportion of A and T residues upstream of all tRNA genes; (2) a region of low duplex stability about 30-35 bp before the coding sequence, often containing a TATA-box like motif; (3) a CAA triplet in the region of the presumed transcription start. The effect of replacement of the AT-rich upstream sequences with GC-rich sequences or unrelated AT-rich sequences was tested by progressive deletions and by inserting randomly cloned sequences upstream of the tRNA gene. GC-rich 5'-flanking sequences were found to be generally incompatible with high levels of expression. The TATA-box like motifs and the CAA triplet were removed or altered by deletion or directed mutagenesis. Mutation of the CAA triplet significantly decreased expression of the tRNA(Leu) gene, suggesting that this CAA triplet is important for transcription efficiency, but mutation or elimination of the TATA-box like motifs generally had little effect. The presence or absence of each of these features in tRNA genes from other organisms is discussed; there are clear and interesting differences between plant tRNA genes and those of yeast and mammals.

Characterization of some major identity elements in plant alanine and phenylalanine transfer RNAs

Alanine and phenylalanine tRNA sequences were amplified by PCR from Arabidopsis thaliana nuclear DNA using degenerate oligonucleotides which introduced specific mutations into the acceptor stem. The aminoacylation of T7 RNA polymerase transcripts of these sequences was investigated in vitro using partially purified bean alanyl- or phenylalanyl-tRNA synthetase. In parallel, the in vivo activity of amber suppressor derivatives of these tRNAs was investigated in transient expression assays in tobacco protoplasts using a beta-glucuronidase (GUS) reporter gene containing a premature amber stop codon. The results show that mutation of the G3:U70 base pair to G3:C70 blocks aminoacylation of plant alanine tRNA, whilst conversion of the G3:C70 pair normally found in plant tRNA(Phe) to G3:U70 enables the mutated tRNA(Phe) to be a good substrate for alanyl-tRNA synthetase and impairs its aminoacylation with phenylalanine. In addition, the amber suppressor derivative of wild-type tRNA(Phe) showed very little suppressor activity in vivo, and was poorly aminoacylated with phenylalanine in vitro, suggesting that the anticodon is a major identity determinant for tRNA(Phe) in plant cells.

Transfer RNA-mediated suppression of stop codons in protoplasts and transgenic plants

We have developed a simple, rapid and sensitive assay for tRNA gene expression in plant cells. A plant tRNA(Leu) gene was site-specifically mutated to encode each of the three anticodon sequences (CUA, UUA and UCA) that recognize, respectively, the amber, ochre and opal stop codons. The suppression activity of these genes was detected by their ability to restore transient beta-glucuronidase (GUS) expression in tobacco protoplasts electroporated with GUS genes containing premature stop codons. Protoplasts co-electroporated with the amber suppressor tRNA gene and a GUS gene containing a premature amber stop codon showed up to 20-25% of the activity found in protoplasts transfected with the functional control GUS gene. Ochre and opal suppressors presented maximum efficiencies of less than 1%. This system could be adapted to examine transcription, processing or aminoacylation of tRNAs in plant cells. In addition, phenotypically normal, fertile tobacco plants expressing a stably incorporated amber suppressor tRNA gene have been obtained. This suppressor tRNA can be used to transactivate a target gene containing a premature amber stop codon by a factor of at least several hundred-fold.

Regulatory studies of L-glutamate dehydrogenase from Trypanosoma cruzi epimastigotes

1. The specific activity of NADP-dependent L-glutamate dehydrogenase (GDH) from T. cruzi epimastigotes increased from 0.7 at early log-phase to 1.4 mumol/min/mg of protein at the stationary phase. 2. When T. cruzi cells were incubated in the presence of L-glutamate (0.08%), the GDH had a specific activity of 2.2, much higher than that of cells incubated in the presence of D-glucose (0.08%), which was 1.2 mumol/min/mg of protein. 3. The specific activity of NADP-dependent GDH from cells incubated in the presence of L-glutamate did not vary when the cells were treated with cycloheximide (100 ng/ml) or chloramphenicol (0.5 mg/ml). 4. The activity of the NAD-dependent GDH did not change in any of the situations described above. 5. AMP, ADP, ATP, citrate, isocitrate, oxaloacetate, fructose-1,6-diP, pyruvate, L-proline and L-arginine did not have any effect on the NADP-linked GDH activity. Product inhibition studies were done on the latter GDH activity.