Characterization of an unusual tRNA-like sequence found inserted in a Neurospora retroplasmid.
We characterized an unusual tRNA-like sequence that had been found inserted in suppressive variants of the mitochondrial retroplasmid of Neurospora intermedia strain Varkud. We previously identified two forms of the tRNA-like sequence, one of 64 nt (TRL-64) and the other of 78 nt (TRL-78) containing a 14-nt internal insertion in the anticodon stem at a position expected for a nuclear tRNA intron. Here, we show that TRL-78 is encoded in Varkud mitochondrial (mt)DNA within a 7 kb sequence that is not present in Neurospora crassa wild-type 74 A mtDNA. This 7-kb insertion also contains a perfectly duplicated tRNA(Trp)gene, segments of several mitochondrial plasmids and numerous GC-rich palindromic sequences that are repeated elsewhere in the mtDNA. The mtDNA-encoded copy of TRL-78 is transcribed and apparently undergoes 5'- and 3'-end processing and 3' nucleotide addition by tRNA nucleotidyl transferase to yield a discrete tRNA-sized molecule. However, the 14 nt intron-like sequence in TRL-78, which is missing in the TRL-64 form, is not spliced detectably in vivo or in vitro. Our results show that TRL-78 is an unusual tRNA-like species that could be incorporated into suppressive retroplasmids by the same reverse transcription mechanism used to incorporate mt tRNAs. The tRNA-like sequence may have been derived from an intron-containing nuclear tRNA gene or it may serve some function, like mtRNA. Our results suggest that mt tRNAs or tRNA-like species may be integrated into mtDNA via reverse transcription, analogous to SINE elements in animal cells.
Mohr
S
S
Institute for Cellular and Molecular Biology, Department of Chemistry and Biochemistry and Section of Molecular Genetics and Microbiology, School of Biological Sciences, University of Texas at Austin 78712, USA.
Wanner
L A
LA
Bertrand
H
H
Lambowitz
A M
AM
D001483:Base Sequence; D015139:Blotting, Southern; D003001:Cloning, Molecular; D017931:DNA Primers; D004271:DNA, Fungal / Q000235:genetics; D004272:DNA, Mitochondrial / Q000235:genetics*; D015246:Deoxyribonuclease EcoRI; D014644:Genetic Variation; D007438:Introns / Q000235:genetics; D008928:Mitochondria / Q000235:genetics*; D008969:Molecular Sequence Data; D016254:Mutagenesis, Insertional / Q000235:genetics*; D009491:Neurospora / Q000235:genetics*; D009492:Neurospora crassa / Q000235:genetics; D009690:Nucleic Acid Conformation; D020161:Physical Chromosome Mapping; D010957:Plasmids / Q000235:genetics*; D012331:RNA, Fungal / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D012364:RNA, Transfer, Trp / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014158:Transcription, Genetic
D017931:DNA Primers; D004271:DNA, Fungal; D004272:DNA, Mitochondrial; D012331:RNA, Fungal; D012364:RNA, Transfer, Trp; D015246:Deoxyribonuclease EcoRI