TESTING THE REVERSE‐SPLICING MODEL OF RDNA INTRON ORIGIN

Abstract

Bhattacharya, D., Simon, D., Chakravarty, S. & Huang, J.Department of Biological Sciences and Interdepartmental Genetics Program, University of Iowa, Iowa City, IA 52242 USAGroup I introns are widely distributed in the nuclear ribosomal RNA genes of algae and fungi. Two models are proposed for intron mobility. The first relies on an endonuclease open reading frame that allows intron “homing” at the DNA‐level into homologous allelic sites. The second model proposes mobility at the RNA‐level through reverse‐splicing. Whereas endonuclease‐mediated homing is depen‐dent on a significant sequence requirement (14‐35 nt) at the insertion site, reverse‐splicing requires as little as 4‐6 nt at the 5′ flanking sequence to initiate intron integration. For this reason, reverse‐splicing may play the more important role in intron lateral transfer in the nuclear genome. In this presentation, we test two major predictions of the reverse‐splicing model with introns in the small (SSU) and large subunit (LSU) nuclear rRNA: 1) Group I introns reverse‐splice into rRNA regions which contain a 4‐6 nt 5′‐exon flanking sequence that builds a helix with the intron internal guide sequence required for forward‐ and reverse‐splicing. 2) Introns are non‐randomly distributed, with most of them clustering in regions that are not “hidden” by rRNA tertiary structure. Phylogenetic analyses show that multiple group I introns in the SSU rDNA of lichen fungi have likely originated through reverse‐splicing of one intron into a heterologous site. The reverse‐splicing model is also supported by the observation that both group I and spliceosomal introns are primarily found in highly conserved rRNA regions that map on the surface of the SSU ribosome. These introns show a positive correlation with rRNA sites that form cross‐links with tRNA and mRNA.