The VS catalytic RNA replicates by reverse transcription as a satellite of a retroplasmid.

Abstract

The mitochondria of certain natural isolates of Neurospora contain both the Varkud plasmid, which encodes a reverse transcriptase, and a small unrelated RNA (VS RNA) that performs RNA-mediated self-cleavage and ligation reactions. Here, we show that VS RNA is transcribed from a VS plasmid DNA template by the Neurospora mitochondrial RNA polymerase using a promoter located immediately upstream of the RNA self-cleavage site that generates monomeric transcripts. VS RNA is then reverse transcribed by the Varkud plasmid reverse transcriptase to yield a full-length (-) strand cDNA, a predicted replication intermediate. Combined with previous genetic evidence, our results indicate that the VS plasmid replicates by reverse transcription as a satellite of the Varkud plasmid. This mode of replication, unprecedented for a satellite RNA, likely reflects the promiscuity of the Varkud plasmid reverse transcriptase, which does not require a specific primer to initiate cDNA synthesis. Our findings indicate how primitive reverse transcriptases with similar relaxed specificity could have facilitated the evolution of new retroelements.

Ribozymes: the characteristics and properties of catalytic RNAs

“…The VS RNA is transcribed from circular or multimeric VS plasmid DNA by a mitochondrial RNA polymerase, and the resulting transcripts are subsequently site-speci¢cally cleaved and ligated to form circular, 881 nucleotides long, RNA monomers [89]. These monomers are then reverse transcribed and made double stranded to form the mature VS plasmid.…”

The Varkud satellite ribozyme

“…The Varkud satellite (VS) RNA is an abundant transcript from DNA found in the mitochondria of a number of natural isolates of Neurospora. Collins and coworkers found that the VS RNA contains an element capable of self-cleavage (Saville and Collins 1990), which is thought to act in the processing of replication intermediates (Kennell et al 1995). At ∼150 nucleotides, the VS ribozyme is the largest of the nucleolytic ribozymes, and is now the only member of this group for which there is no crystal structure.…”

Identification of functional domains in theself-cleaving Neurospora VS ribozyme using damage selection

“…It is transcribed by mitochondrial RNA polymerase from a double-stranded plasmid, VS DNA, and replicates by reverse transcription as a satellite of another plasmid (Kennell et al, 1995). VS RNA is capable of self-cleavage and self-ligation reactions (Saville & Collins, 1990 presumably as part of its replication cycle.…”