Influence of the relA gene on ribosome frameshifting.

We have examined the influence of genotype at the relA locus on the kinetics of leftward (or -1) frameshifting at a variety of codons calling for a limiting aminoacyl-tRNA species. We used lacZ left-frameshift reporter constructs carrying the sequenceU UUC XYZ, whereXYZ was each of three triplets coding for three different amino acids; we slowed the ribosomes at each of these by limiting for the amino acid or for the aminoacyl-tRNA. In all cases, limitation stimulated leftward frameshifting. In all cases, the stimulation was greater in relA mutant cells than in their wild-type relA(+) counterparts. In the latter genotype, the increased frameshifting was constant from the start of the limitation regime. This was also true of the relA mutant strain during limitation for lysine-tRNA or for leucine; however, during limitation for isoleucine-tRNA (or for isoleucine) the mutant showed a gradual, progressive increase in frameshifting, suggesting an indirect effect. We suggest that gradual accumulation of undermodified tRNAs, which is characteristic of the relA response, is involved. However, the specific modification involved is unknown. It is not queosine: analysis of a tgt mutant that is completely defective in queosine modification showed no increase in leftward frameshifting on the reporter which showed the larger, gradual increase during the relA response to isoleucine-tRNA limitation.

Point mutations in the mitochondrial tRNA(Lys) gene: implications for pathogenesis and mechanism.

MERRF (myoclonic epilepsy with ragged-red fibers) is a severe, multisystem disorder characterized by myoclonus, seizures, progressive cerebellar syndrome, muscle weakness, and the presence of ragged-red fibers in the muscle biopsy. MERRF is associated with heteroplasmic point mutations, either A8344G or T8356C, in the gene encoding the mitochondrial tRNA(Lys). The human rho degree cell system was utilized to examine the phenotypic consequences of these mutations, and to investigate their molecular genetic causes. Wild-type and mutant transmitochondrial cell lines harboring a pathogenic point mutation at either A8344G or T8356C in the human mitochondrial tRNA(Lys) gene were isolated and examined. Mitochondrial transformants containing 100% mutated mitochondrial DNAs (mtDNAs) exhibited severe defects in respiratory chain activity, in the rates of protein synthesis, and in the steady-state levels of mitochondrial translation products as compared with mitochondrial transformants containing 100% wild-type mtDNAs. In addition, both mutant cell lines exhibited the presence of aberrant mitochondrial translation products. These results demonstrate that two different mtDNA point mutations in tRNA(Lys) result in fundamentally identical defects at the cellular level, and that these specific protein synthesis abnormalities contribute to the pathogenesis of MERRF.

In vitro analysis of mutations causing myoclonus epilepsy with ragged-red fibers in the mitochondrial tRNA(Lys)gene: two genotypes produce similar phenotypes.

Cytoplasts from patients with myoclonus epilepsy with ragged-red fibers harboring a pathogenic point mutation at either nucleotide 8344 or 8356 in the human mitochondrial tRNA(Lys) gene were fused with human cells lacking endogenous mitochondrial DNA (mtDNA). For each mutation, cytoplasmic hybrid (cybrid) cell lines containing 0 or 100% mutated mtDNAs were isolated and their genetic, biochemical, and morphological characteristics were examined. Both mutations resulted in the same biochemical and molecular genetic phenotypes. Specifically, cybrids containing 100% mutated mtDNAs, but not those containing the corresponding wild-type mtDNAs, exhibited severe defects in respiratory chain activity, in the rates of protein synthesis, and in the steady-state levels of mitochondrial translation products. In addition, aberrant mitochondrial translation products were detected with both mutations. No significant alterations were observed in the processing of polycistronic RNA precursor transcripts derived from the region containing the tRNA(Lys) gene. These results demonstrate that two different mtDNA mutations in tRNA(Lys), both associated with the same mitochondrial disorder, result in fundamentally identical defects at the cellular level and strongly suggest that specific protein synthesis abnormalities contribute to the pathogenesis of myoclonus epilepsy with ragged-red fibers.

tRNA processing in human mitochondrial disorders.

Many human mitochondrial disorders are associated with mutations in tRNA genes or with deletions of regions containing tRNA genes, all of which may be suspected to play a role in recognition by RNase P. Here we describe the analysis of five such mutations. The results presented here demonstrate that none of these mutations result in errors in RNase P function. Further studies of mutations in tRNAs need to be pursued to elucidate the identity elements for RNase P function in mammalian mitochondria.

A proposed mechanism for promoter-associated DNA rearrangement events at a variant surface glycoprotein gene expression site.

The expressed variant cell surface glycoprotein (VSG) gene of the protozoan parasite Trypanosoma brucei is invariably found at one of several telomeric VSG gene expression sites (ESs). The active ES in variant 118 clone 1 is found on a 1.5-Mb chromosome, and the promoter region is located more than 45 kb upstream of the VSG gene. We had previously shown that DNA rearrangement events occurred in the promoter region, specifically at inactivation of this ES (K. M. Gottesdiener, H.-M. Chung, S. L. Brown, M. G.-S. Lee, and L. H. T. Van der Ploeg, Mol. Cell. Biol. 11:2467-2477, 1991). In this report, we describe the cloning of the entire 17-kb promoter region, which revealed the presence of two identical 2.15-kb tandem promoter repeats separated by 13 kb of DNA. The two virtually identical promoter repeats both function efficiently in directing transcription in transient transfection assays in insect-form trypanosomes. We characterized the DNA rearrangement events that occur at ES inactivation, and by studying both of the reciprocal products of this recombination event, we infer that these result from direct (promoter) repeat recombination, formation of heteroduplex DNA, and a reciprocal exchange event that releases a circular DNA as a side product of the reaction. The finding of DNA recombinational events in a region of the VSG gene ES that encodes the promoter(s), and their relatively frequent occurrence at ES inactivation, suggests a possible role in ES control.