Mobile intron RNA from a bacterial predator accumulates in dead archaeal cells

Introns in prokaryotes are classified as mobile genetic elements normally located within essential genes. Their RNA transcripts are functional if spliced by the intron´s ribozyme activity. The excised intron RNA is assumed to proliferate by leaving the host cell and being integrated into foreign genomes. Here we show the presence of a bacterial intron´s RNA in archaeal cells. The predatory ultramicrobial epibiont Candidatus Velamenicoccus archaeovorus contains a group I intron in its 23 S ribosomal RNA gene. In search for predation evidence, we visualized the intron RNA by in situ hybridization and observed it in dead cells of the filamentous archaeon Methanothrix soehngenii in a slowly growing methanogenic enrichment culture on limonene. Transcriptome analysis detected a ratio of one unspliced transcript or intron molecule per 20,000 mature 23 S rRNA molecules. Our study visualized that spliced intron RNA can leave the host and enter foreign cells. Spliced intron RNA adds to extracellular RNA, so far known to function as intercellular signaling molecules and metabolic inhibitors.