The I‐Ceu I endonuclease: purification and potential role in the evolution of Chlamydomonas group I introns

Abstract

During genetic crosses between the interfertile green algae Chlamydomonas eugametos and Chlamydomonas moewusii, the I‐Ceul endonuclease encoded by the fifth group I intron (CeLSU · 5) in the C. eugametos chloroplast large subunit rRNA gene mediates the mobility of this intron by introducing a double‐strand break near the insertion site of the intron in the corresponding C. moewusii intronless allele. To characterize the biochemical properties of this endonuclease, we have purified I‐CeuI and determined the optimal reaction conditions for cleavage. I‐CeuI activity is maximal at 50°C, pH 10.0, 2.5 mM MgCl₂ and in the absence of NaCl. Unlike the well‐characterized I‐SceI endonuclease, I‐CeuI remains stable following preincubation in the absence of substrate. We discuss the role that homing endonucleases may have played in the evolution of Chlamydomonas chloroplast group I introns.