Intron open reading frames as mobile elements and evolution of a group I intron

Group I introns are proposed to have become mobile following theacquisition of open reading frames (ORFs) that encode highly specific DNAendonucleases. This proposal implies that intron ORFs could behave asautonomously mobile entities. This was supported by abundant circumstantialevidence but no experiment of ORF transfer from an ORF- containing intronto its ORF-less counterpart has been described. In this paper we presentsuch experiments, which demonstrate the efficient mobility of themitochondrial nad1-i4-orf1 between two Podospora strains. The homing ofthis mobile ORF was accompanied by a bidirectional co-conversion that didnot systematically involve the whole intron sequence. Orf1 acquisitionwould be the most recent step in the evolution of the nad1-i4 intron, whichhas resulted in many strains of Podospora having an intron with two ORFs(biorfic) and four splicing pathways. We show that two of the splicingevents that operate in this biorfic intron, as evidenced by PCRexperiments, are generated by a 5'-alternative splice site, which is mostprobably a remnant of the monoorfic ancestral form of the intron. Wepropose a sequential evolution model that is consistent with the fourorganizations of the corresponding nad1 locus that we found among variousspecies of the Pyrenomycete family; these organizations consist of nointron, an intron alone, a monoorfic intron, and a biorfic intron.