Template switching by a group II intron reverse transcriptase: biochemical analysis and implications for RNA-seq

The reverse transcriptases (RTs) encoded by mobile group II intron and other non-LTR-retro-elements differ from retroviral RTs in being able to template switch from the 5' end of one template to the 3' end of another without pre-existing complementarity between the donor and acceptor nucleic acids. Here, we used the ability of a thermostable group II intron RT (TGIRT) to template switch directly from synthetic RNA template/DNA primer duplexes having either a blunt end or a 3'-DNA overhang end to establish a complete kinetic framework for the reaction and identify conditions that more efficiently capture acceptor RNAs or DNAs. The rate and amplitude of template switching are optimal from starter duplexes with a single nucleotide 3'-DNA overhang complementary to the 3' nucleotide of the acceptor RNA, suggesting a role for non-templated nucleotide addition of a complementary nucleotide to the 3’ end of cDNAs synthesized from natural templates. Longer 3'-DNA overhangs progressively decrease the rate of template switching, even when complementary to the 3' end of the acceptor template. Although dependent upon only a single base pair between the donor and acceptor, template switching discriminates against mismatches, which coupled with the high processivity of the enzyme, enables the synthesis of full-length DNA copies of acceptor nucleic acids beginning directly at their 3' end. We discuss possible biological functions of the template-switching activity of group II intron and other non-LTR-retroelements RTs, as well as the optimization of this activity for adapter addition in RNA-and DNA-seq. A group II intron RT was used to template switch and reverse transcribe synthetic RNA oligonucleotides and the resulting products were sequenced by Illumina Nextseq 500, and the junctions between acceptor and donor RNA molecules were analyzed.