family of nuclear homing endonucleases

We have discovered two new group I intron elements from nuclear extrachromosomal rDNA that bears an apparent similarity to PpLSU3. The myxomycete Didymium iridis has a mobile group I intron that encodes a putative homing endonuclease I-Dir of 29 kd, whereas the amoeba-flagellate Naegleria andersoni ssp andersoni contains a similar intron that encodes the putative homing endonuclease I-Naa of 28 kd. I-Dir, I-Naa and I-Ppo are all basic proteins with a theoretical isoelectric point of 9.5, 9.9 and 8.2, respectively. Furthermore, we could not identify any significant sequence similarity between these proteins and proteins in the GenBank and EMBL libraries, and they do not contain any of the sequence motifs (eg. LAGLI-DADG or GIY-YIG) found in other homing endonucleases. However, we have identified a conserved region among the nuclear homing endonucleases which consists of several His and Cys residues.