THE VS CATALYTIC RNA REPLICATES BY REVERSE TRANSCRIPTION AS A SATELLITE OF A RETROPLASMID

The mitochondria of certain natural isolates of Neurospora contain bot h the Varkud plasmid, which encodes a reverse transcriptase, and a sma ll unrelated RNA (VS RNA) that performs RNA-mediated self-cleavage and ligation reactions. Here, we show that VS RNA is transcribed from a V S 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 tra nscribed by the Varkud plasmid reverse transcriptase to yield a full-l ength (-) strand cDNA, a predicted replication intermediate. Combined with previous genetic evidence, our results indicate that the VS plasm id replicates by reverse transcription as a satellite of the Varkud pl asmid. This mode of replication, unprecedented for a satellite RNA, li kely reflects the promiscuity of the Varkud plasmid reverse transcript ase, which does not require a specific primer to initiate cDNA synthes is. Our findings indicate how primitive reverse transcriptases with si milar relaxed specificity could have facilitated the evolution of new retroelements.