| In nearly all sexual eukaryotes, the mating progeny receive most of their chloroplast and mitochondrial genomes from a single parent. For example, in mammals, mitochondria from the paternal parent appears to be quickly destroyed in the zygote, and in most angiosperms, the plastids are inherited exclusively from the female parent. The mechanisms proposed include physical exclusion of the plastids from the sperm cell and degeneration/elimination of plastids during the mating process. In C. reinhardtii, uniparental inheritance is based on selective degradation of the cpDNA from the minus parent in the early zygote, while the cpDNA from the plus parent is protected from degradation. The fate of cpDNA in the zygote is therefore under the control of the MT loci, each spanning ~300 kb. The OTU2 gene encodes a homolog of otubain, a cysteine protease that functions in the deubiquitination pathway. The OTU2 gene resides in the "Rearranged"(R) domain of the MT locus; its ex
pression is confined to gametes and continues at least 2 hr into zygote development. Ectopic expression of MT+ encoded Otu2-p (Otu2-plus) protein in minus gametes affects the cpDNA inheritance pattern, resulting in increased biparental progeny by protecting minus cpDNA from degradation in zygotes. By contrast, the protein encoded in the MT- locus, Otu2-m (Otu2-minus), was not able to exercise the protection function. Domain-swapping experiments localized the region that provides the protection function of Otu2-p to the C-terminal region. Mutating the Cys residue in the conserved protease domain abolishes the cpDNA protection conferred by Otu2-p, suggesting that cysteine protease activity is required for its function. Currently we are localizing Otu2 with Otu2-specific antibody and fluorescent protein tags in Chlamydomonas cells. We suggest that Otu2 is a strong molecular candidate for involvement in uniparental inheritance. |