ENHANCEMENT OF SOMATIC INTRACHROMOSOMAL HOMOLOGOUS RECOMBINATION IN ARABIDOPSIS BY THE HO ENDONUCLEASE

The HO endonuclease promotes gene conversion between mating-type allel es in yeast by a DNA double-strand break at the site of conversion (th e MAT-Y/Z site). As a first step toward understanding the molecular ba sis of homologous recombination in higher plants, we demonstrate that expression of HO in Arabidopsis enhances intrachromosomal recombinatio n between inverted repeats of two defective beta-glucuronidase (gus) g enes (GUS(-) test construct). One of these genes has the Y/Z site. The two genes share 2.5 kb of DNA sequence homology around the HO cut sit e. Somatic recombination between the two repeats was determined by usi ng a histochemical assay of GUS activity. The frequency of Gus(+) sect ors in leaves of F-1 plants from a cross between parents homozygous fo r the GUS(-) test construct and HO, respectively, was in-fold higher t han in F-1 plants from a cross between the same plant containing the G US(-) test construct and a wild-type parent. Polymerase chain reaction analysis showed restoration of the 5' end of the GUS gene in recombin ant sectors. The induction of intrachromosomal gene conversion in Arab idopsis by HO reveals the general utility of site-specific DNA endonuc leases in producing targeted homologous recombination in plant genomes .