부산대학교 도서관

The two group I introns Nae.L1926 and Nmo.L2563, 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 Ppo.L1925 (PpLSU3) with ORFs extending the L1-loop of a typical group IC1 ribozyme. Nae.L1926, Nmo.L2563 and Ppo.L1925 RNAs all self-splice in vitro , generating ligated exons and full-length intron circles as well as internal processed excised intron RNAs. Formation of full-length intron circles is found to be a general feature in RNA processing of ORF-containing nuclear group I introns. Both Naegleria LSU rDNA introns contain a conserved polyadenylation signal at exactly the same position in the 3′ end of the ORFs close to the internal processing sites, indicating an RNA polymerase II-like expression pathway of intron proteins in vivo . The intron proteins I-Nae I and I-Nmo I encoded by Nae.L1926 and Nmo.L2563, respectively, correspond to His-Cys homing endonucleases of 148 and 175 amino acids. I-Nae I contains an additional sequence motif homologous to the unusual DNA binding motif of three antiparallel β sheets found in the I-Ppo I endonuclease, the product of the Ppo.L1925 intron ORF. [ABSTRACT FROM AUTHOR]