The mobility (homing) of the yeast mitochondrial DNA group II intron al2 occurs via target DNA-primed reverse transcription at a double-strand break in the recipient DNA. Here, we show that the site-specific DNA endonuclease that makes the double-strand break is a ribonucleoprotein complex containing the al2-encoded reverse transcriptase protein and excised a12 RNA. Remarkably, the al2 RNA catalyzes cleavage of the sense strand of the recipient DNA, while the a12 protein appears to cleave the antisense strand. The RNA-catalyzed sense strand cleavage occurs via a partial reverse splicing reaction in which the protein component stabilizes the active intron structure and appears to confer preference for DNA substrates. Our results demonstrate a biologically relevant ribozyme reaction with a substrate other than RNA. © 1995.
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