RNA processing in Neurospora crassa mitochondria: use of transfer RNA sequences as signals.

Abstract

We have used RNA gel transfer hybridization, S1 nuclease mapping and primer extension to analyze transcripts derived from several genes in Neurospora crassa mitochondria. The transcripts studied include those for cytochrome oxidase subunit III, 17S rRNA and an unidentified open reading frame. In all three cases, initial transcripts are long, include tRNA sequences, and are subsequently processed to generate the mature RNAs. We find that endpoints of the most abundant transcripts generally coincide with those of tRNA sequences. We therefore conclude that tRNA sequences in long transcripts act as primary signals for RNA processing in N. crassa mitochondria. The situation is somewhat analogous to that observed in mammalian mitochondrial systems. The difference, however, is that in mammalian mitochondria, noncoding spacers between tRNA, rRNA and protein genes are very short and in many cases non‐existent, allowing no room for intergenic RNA processing signals whereas, in N. crassa mtDNA, intergenic non‐coding sequences are usually several hundred nucleotides long and contain highly conserved GC‐rich palindromic sequences. Since these GC‐rich palindromic sequences are retained in the processed mature RNAs, we conclude that they do not serve as signals for RNA processing.

The analysis of the complete mitochondrial genome of Lecanicillium muscarium (synonym Verticillium lecanii) suggests a minimum common gene organization in mtDNAs of Sordariomycetes: phylogenetic implications

“…1). As has been suggested previously in other fungi (Breitenberger et al, 1985;Schafer, 2003), the single tRNA genes may have acted as sites of genetic recombination, and therefore genetic rearrangements. Recombination at the trn genes may explain the differences in mtDNA gene order of N. crassa compared with L. muscarium and H. jecorina.…”

Processing of precursor RNAs from mitochondria ofNeurospora crassa

“…The predicted secondary structures of the two intergenic regions and of the region downstream of the CO II gene are shown in figs. [8][9][10]. The secondary structures are very complex as a result of multiple palindromic sequences, some of which are stabilized by so-called pseudoknots.…”

“…The long transcripts are considered to be precursor RNAs from which the functionally active RNAs arise by a series of processing steps. Presumably tRNA sequences are used as recognition sites in the processing of precursors (8)(9)(10). For instance the messengers for subunit 1 of cytochrome aa3 and for apocytochrome b are excised from a common precursor by cutting near the acceptor stem of adjacent tRNA sequences (9)(10).…”

Transcript mapping and processing of mitochondrial RNA in the chlorophyte alga Prototheca wickerhamii

“…In Neurospora, Aspergillus and Schizosaccharomyces mitochondria, polycistronic transcripts were found to be processed by tRNA excision and additional processing events, generating mono-or dicistronic RNA species (e.g. [26,16,14,1,45,8]). In Kluyveromyces and Saccharomyces, multiple promoters exist for several small transcriptional units [63,22], and processing occurs by endonucleolytic cleavage close to tRNAs or at conserved sequence motifs [60,12].…”

“…For example, compact animal mtDNAs are transcribed into precursor molecules, which subsequently are processed to monocistronic RNAs by excision of individual tRNAs [58,59]. A similar mode of RNA processing seems to be employed in mitochondria of Aspergillus and Neurospora, where tRNA are used as processing signals [16,14,1,26 ]. However, mitochondrial DNA from the yeast Saccharomyces cerevisiae carries multiple transcription units, which are controlled by their own promoters [22].…”