Abstract
How group I introns originate in nuclear ribosomal (r)RNA genes is an important question in evolutionary biology. Central to this issue is the multitude of group I introns present in evolutionarily distantly related plant, fungal, and protist lineages, together with an understanding of their origin and lateral transfer from one exon to another, between cell organelles, and between cells. These introns vary considerably in primary and secondary structure; and their provenance from a few or perhaps many mobile elements that have spread in rRNAs is unknown. Here we show that a novel lineage of group IC1 introns inserted at position 516 (Escherichia coli gene numbering) in the small subunit rRNA in bangiophyte red algae and a brown alga (Aureoumbra lagunensis) are specifically related, although their host cells are not. These bangiophyte and Aureoumbra introns are the only known cases that have a helical insertion in the P5b helix. The highly conserved primary and secondary structure of the extra P5b helix suggests that it is important, although its specific function is unknown. Our study attempts to understand the origin and movement of these IC1 introns.