In vitro synthesis of multicopy single-stranded DNA, using separate primer and template RNAs, by Escherichia coli reverse transcriptase.

A minor population of wild strains of Escherichia coli contains a retron, a retroelement responsible for the synthesis of multicopy single-stranded DNA (msDNA). The retron is a genetic element consisting of the gene for reverse transcriptase (RT) and the msr-msd region under a single promoter. A single RNA transcript from the msr-msd region serves not only as a template but also as a primer for msDNA synthesis. Here, using a cell-free system with purified RT from retron Ec73, we examined whether the reaction can occur in a bimolecular reaction with use of separately expressed msr and msd transcripts. DNA sequencing of the cell-free product revealed that the sequence of the 5'-end region was identical to that of msDNA-Ec73, indicating that the cDNA synthesis was primed from the 2'-OH group of the specific internal G residue of the primer RNA, identical to the branching G residue in the RNA molecule of msDNA-Ec73. The present results raise an intriguing possibility for a role of bacterial retrons in vivo, the possibility that cellular mRNAs can be converted into cDNAs in retron-harboring cells if the mRNAs contain a sequence complementary to the sequence directly upstream of the branching G residue of the msr RNA transcript.