Excision of micronuclear-specific DNA requires parental expression of Pdd2p and occurs independently from DNA replication in Tetrahymena thermophila

Elimination of germ-line DNA segments is an essential step in the somatic d evelopment of many organisms and in the terminal differentiation of several specialized cell types. In binuclear ciliates, including Tetrahymena therm ophila, DNA elimination occurs during the conversion of the germ-line micro nuclear genome into the somatic genome of the new macronucleus. Little is k nown about molecular determinants and regulatory mechanisms involved in thi s process. Pdd2p is one of a small set of Tetrahymena polypeptides whose ti me of synthesis, nuclear localization, and physical association with sequen ces destined for elimination suggest an involvement in the DNA elimination process. In this study, we report that loss of parental expression of Pdd2p leads to the perturbation of several DNA rearrangement processes in develo ping zygotic macronuclei, including excision of internal eliminated sequenc es, excision of chromosome breakage sequences, and endoreplication of the n ew macronuclear genome and eventually results in lethality of the progeny. We demonstrate that excision and elimination of micronuclear-specific DNA o ccurs independently of endoreplication of the new macronuclear genome that takes place during the same period of time. Thus, our data indicate that pa rental expression of Pdd2p is required for successful DNA elimination and d evelopment of somatic nuclei.