The two group I introns and , found at two different sites in nuclear LSU rRNA genes of Naegleria amoebo-flagellates, have been characterized in vitro. Their structural organization is related to that of the mobile Physarum intron (PpLSU3) with ORFs extending the L1-loop of a typical group IC1 ribozyme. , and RNAs all selfsplice in vitro, generating ligated exons and full-length intron circles as well as internal processed excised intron RNAs