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Nucleotide sequence of the Varkud mitochondrial plasmid of Neurospora and synthesis of a hybrid transcript with a 5′ leader derived from mitochondrial RNA
Ist Teil von
Journal of molecular biology, 1988-11, Vol.204 (1), p.1-25
Ort / Verlag
Oxford: Elsevier Ltd
Erscheinungsjahr
1988
Quelle
Access via ScienceDirect (Elsevier)
Beschreibungen/Notizen
The Mauriceville and Varkud mitochondrial plasmids of
Neurospora are closely related, closed circular DNAs (3.6 and 3.7 kb, respectively; 1 kb = 10
3 bases or base-pairs), whose characteristics suggest relationships to mitochondrial DNA introns and retrotransposons. Here, we characterized the structure of the Varkud plasmid, determined its complete nucleotide sequence and mapped its major transcripts. The Mauriceville and Varkud plasmids have more than 97% positional identity. Both plasmids contain a 710 amino acid open reading frame that encodes a reverse transcriptase-like protein. The amino acid sequence of this open reading frame is strongly conserved between the two plasmids (
701
710
amino acids) as expected for a functionally important protein. Both plasmids have a 0.4 kb region that contains five
PstIpalindromes and a direct repeat of approximately 160 base-pairs. Comparison of sequences in this region suggests that the Varkud plasmid has diverged less from a common ancestor than has the Mauriceville plasmid. Two major transcripts of the Varkud plasmid were detected by Northern hybridization experiments: a full-length linear RNA of 3.7 kb and an additional prominent transcript of 4.9 kb, 1.2 kb longer than monomer plasmid. Remarkably, we find that the 4.9 kb transcript is a hybrid RNA consisting of the full-length 3.7 kb Varkud plasmid transcript plus a 5′ leader of 1.2 kb that is derived from the 5′ end of the mitochondrial small rRNA. This and other findings suggest that the Varkud plasmid, like certain RNA viruses, has a mechanism for joining heterologous RNAs to the 5′ end of its major transcript, and that, under some circumstances, nucleotide sequences in mitochondria may be recombined at the RNA level.