| In Chlamydomonas , uniparental (UP) inheritance of chloroplast DNA (cpDNA) entails the selective degradation, by an unidentified nuclease, of the cpDNA contributed to the early zygote from the mating-type minus (mt-) parent. The cpDNA from the mating-type plus (mt+) parent is somehow protected from degradation and is transmitted to all 4 meiotic progeny. Sager discovered that low levels of UV irradiation of plus gametes prior to mating increased the percentage of "biparental" (BP) progeny inheriting both mt+ and mt- copies, while irradiation of minus gametes was without effect, suggesting that plus gametes contribute a UV-sensitive component/process to cpDNA degradation. Our lab has previously identified a gene cluster, EZY2, without homologues in the database, that is encoded in the MT+ but not the MT- locus. EZY2 is not expressed in plus gametes but is expressed immediately after zygote formation, dramatically induced within 30 min, and reduced by 2 hr, concurrent with the time frame during which mt- cp-DNA is destroyed. We show here that the Ezy2 protein localizes to the chloroplast, as predicted by its transit sequence. We also show that the expression of EZY2 is selectively and completely inhibited when plus, but not minus, gametes are irradiated with UV prior to mating. Hence Ezy2 is a strong candidate for involvement in the destruction of mt- chloroplast DNA in zygotes. In ongoing experiments, we are introducing ectopically-expressed EZY2 into minus cell lines, and will mate them with UV-irradiated plus gametes. If Ezy2 is necessary for destruction, then the cross should yield UP rather than BP cpDNA inheritance.
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