Genetic Analysis of the Chlamydomonas Reinhardtii I-Crei Mobile Intron Homing System in Escherichia Coli

We have developed and used a genetic selection system in Escherichia coli to study functional requirements for homing site recognition and cleavage by a representative eukaryotic mobile intron endonuclease. The homing endonuclease, I-CreI, was originally isolated from the chloroplast of the unicellular green alga Chlamydomonas reinhardtii. I-CreI homing site mutants contained base pair substitutions or single base deletions that altered the rate of homing site cleavage and/or product release. I-CreI endonuclease mutants fell into six phenotypic classes that differed in in vivo activity, toxicity or genetic dominance. Inactivating mutations clustered in the N-terminal 60% of the I-CreI amino acid sequence, and two frameshift mutations were isolated that resulted in premature translation termination though retained partial activity. These mutations indicate that the N-terminal two-thirds of the I-CreI endonuclease is sufficient for homing site recognition and cleavage. Substitution mutations altered in four potential active site residues were examined: D20N, Q47H or R70A substitutions inactivated endonuclease activity, whereas S22A did not. The genetic approach we have taken complements phylogenetic and structural studies of mobile intron endonucleases and has provided new information on the mechanistic basis of I-CreI homing site recognition and cleavage.