Properties of a discrete high molecular weight poly (A) containing mitochondrial RNA

Apocytochrome b and other mitochondrial DNA sequences are differentially expressed during the life cycle of Trypanosoma brucei

Cytochromes and Krebs cycle enzymes are not detected in bloodstream forms of Trypanosoma brucei but are present in procyclic forms. We have analyzed transcription of mitochondrial sequences which contain the apocytochrome b gene and several other open reading frames (ORFs). Multiple transcripts map to individual DNA sequences located on both DNA strands. Larger low abundance transcripts map to multiple ORFs and may be precursor RNAs. Small abundant transcripts map to G + C rich sequences that do not have obvious protein coding functions. The larger of two presumptive apocytochrome b transcripts is strikingly more abundant in procyclic than bloodstream forms and other mitochondrial transcripts are also differentially abundant between these two forms. In addition, many mitochondrial transcripts appear to be differentially polyadenylated between bloodstream and procyclic forms. We suggest that the mechanisms which regulate the production of the mitochondrial respiratory system in T. brucei involve differential expression of mitochondrial genes.

Sequences around the 3'-end of a ribosomal RNA gene of hamster mitochondria. Further support for the 'transcriptional attenuation' model

We have determined DNA sequences near and beyond the end of the large ribosomal subunit RNA gene of hamster mitochondria. They indicate that (1) at least 20% of the rRNA transcripts terminate beyond the gene, in the downstream tRNAleu gene; (2) a transcription-attenuator-like structure occurs just upstream from the end of the rRNA gene; and (3) a conserved reading frame (' URF1 ') has an unusual initiation codon, GTG, in rodent mitochondria.

Oligoadenylate is present in the mitochondrial RNA of Saccharomyces cerevisiae

We examined Saccharomyces cerevisiae mitochondrial RNA for polyadenylate. Using hybridization to [3H]polyuridylate as the assay for adenylate sequences, we found adenylate-rich oligonucleotides approximately 8 residues long. Longer polyadenylate was not detected. Most of the adenylate-rich sequence is associated with the large mitochondrial rRNA. The remainder is associated with the 10-12S group of transcripts.

Oligoadenylate is present in the mitochondrial RNA of Saccharomyces cerevisiae

We examined Saccharomyces cerevisiae mitochondrial RNA for polyadenylate. Using hybridization to [3H]polyuridylate as the assay for adenylate sequences, we found adenylate-rich oligonucleotides approximately 8 residues long. Longer polyadenylate was not detected. Most of the adenylate-rich sequence is associated with the large mitochondrial rRNA. The remainder is associated with the 10-12S group of transcripts.

tRNA punctuation model of RNA processing in human mitochondria

A 3'-end proximal segment of most of the putative mRNAs encoded in the heavy strand of HeLa cell mtDNA has been partially sequences and aligned with the DNA sequence. In all cases, the 3'-end nucleotide of the individual mRNA coding sequences has been found to be immediately contiguous to a tRNA gene or another mRNA coding sequence. These and previous results indicate that the heavy (H) strand sequences coding for the rRNA, poly(A)-containing RNA and tRNA species form a continuum extending over almost the entire length of this strand. We propose that the H strand is transcribed into a single polycistronic RNA molecule, which is processed later into mature species by precise endonucleolytic cleavages which occur, in most cases, immediately before and after a tRNA sequence.

The 3' terminus of the large ribosomal subunit ("17S") RNA from hamster mitochondria is ragged and oligoadenylated

The 3'-terminal sequence of the large ribosomal subunit ("17S") RNA of hamster mitochondria has been examined by means of oligonucleotide fingerprint analysis of 3' end-labeled samples. Patterns from partial acid or alkaline hydrolysates indicated marked heterogeneity and suggested an array of termini that included GGUUAOH, AnAOH and AnGOH (where n is about 10). Patterns from samples subjected to exhaustive digestion with ribonucleases T1 plus A, or with each separately, confirmed these inferences, and strikingly demonstrated the existence of oligoadenylated families of 3'-terminal sequences. Under the assumption that the oligoadenylate was added posttranscriptionally, these results indicated that the 3'-terminal transcribed moieties consist of variants of YAGGUUOH. Counting from the last U, we inferred that many such sequences end at U1 and G4, while smaller numbers end at G3 and A5; and that major sites of oligoadenylation occur at U1 and G4, and a minor site occurs at A5. This novel combination of imprecise termination of transcription or "sizing" of transcripts, and variable addition of adenylate residues, is discussed with regard to the mechanism of expression of the mammalian mitochondrial ribosomal RNA genes.

The tRNA genes punctuate the reading of genetic information in human mitochondrial DNA

A detailed transcription map of HeLa cell mitochondrial DNA (mtDNA) has been constructed by using the S1 protection technique to localize precisely the sequences coding for the ribosomal RNA (rRNA) and poly(A)-containing species on the physical map of the DNA. This transcription map has been correlated with the positions of the tRNA genes derived from the mtDNA sequence. It has been shown that, with the exception of the D loop and another small segment near the origin of replication, the mtDNA sequences are completely saturated by the rRNAs, poly(A)-containing RNAs and tRNA coded for by the two strands. No evidence for intervening sequences has been found. The sequences coding for the individual poly(A)-containing RNA and rRNA species appear to be immediately contiguous on one side, and most frequently on both sides, to tRNA coding sequences. Furthermore, the H strand sequences coding for the two rRNAs, the poly(A)-containing RNAs and the tRNAs appear to be adjacent to each other, extending from coordinate 2/100 to coordinate 95/100 of the genome relative to the origin taken as 0/100. The results are consistent with a model of transcription of the H strand in the form of a single molecule which is processed into mature RNA species by precise endonucleolytic cleavages, occurring in almost all cases immediately before and after a tRNA sequence. The tRNA sequences may play an important role as recognition signals in the processing of the primary transcripts.