REVERSE-TRANSCRIPTASE ACTIVITY OF AN INTRON ENCODED POLYPEPTIDE

A number of group II introns from eukaryotic organelles and prokaryote s contain open reading frames for polypeptides with homology to retrov iral reverse transcriptases (RTs). We have used the yeast transposon ( Ty) system to express ORFs for RTs from eukaryotic organelles. This in cludes the mitochondrial coxI intron i1 from the fungus Podospora anse rina, the plastid petD intron from the alga Scenedesmus obliquus and t he mitochondrial RTL gene from the alga Chlamydomonas reinhardtii. The ORFs were fused with the TYA ORF from the yeast retrotransposon Ty to produce virus-like particles in the recipient strains with detectable amounts of the RT-like polypeptides. Analysis of the heterologous gen e products revealed biochemical evidence that the P.anserina intron en codes an RNA-directed DNA polymerase with properties typically found f or RTs of viral or retrotransposable origin. In vitro assays showed th at the intron encoded RT is sensitive to RT inhibitors such as N-ethyl maleimide and dideoxythymidine triphosphate but is insensitive against the DNA polymerase inhibitor aphidicolin. The direct biochemical evid ence provided here supports the idea that intron encoded RTs are invol ved in intron transposition events.