Two paralogous PHD finger proteins participate in Paramecium tetraurelia's natural genome editing

The unicellular eukaryote Paramecium tetraurelia contains functionally distinct nuclei: germline micronuclei (MICs) and a somatic macronucleus (MAC). During sexual reproduction, the MIC genome is reorganized into a new MAC genome and the old MAC is lost. Almost 45,000 unique Internal Eliminated Sequences (IESs) located throughout the genome require precise excision to guarantee a functional new MAC genome. Here, we characterize a pair of paralogous PHD finger proteins involved in DNA elimination. DevPF1, the early-expressed paralog, is present in only some of the gametic and post-zygotic nuclei during meiosis. Both DevPF1 and DevPF2 localize in the new developing MACs, where IESs excision occurs. In DevPF2 knockdown (KD), long IESs are preferentially retained and late-expressed small RNAs decrease; no length preference for retained IESs was observed in DevPF1-KD and development-specific small RNAs were abolished. The expression of at least two genes from the new MAC seems to be influenced by DevPF1- and DevPF2-KD. Thus, both PHD fingers are crucial for new MAC genome development, with distinct functionalities, potentially via regulation of non-coding and coding transcription in the MICs and new MACs.