ISWI1 complex proteins facilitate developmental genome editing in Paramecium
Chromatin remodeling is required for essential cellular processes, including DNA replication, DNA repair, and transcription regulation. The ciliate germline and soma are partitioned into two distinct nuclei within the same cell. During a massive editing process that forms a somatic genome, ciliates eliminate thousands of DNA sequences from a germline genome copy in the form of internal eliminated sequences (IESs). Recently we showed that the chromatin remodeler ISWI1 is required for somatic genome development in the ciliate Paramecium tetraurelia. Here we describe two paralogous proteins, ICOP1 and ICOP2, essential for DNA elimination. ICOP1 and ICOP2 are highly divergent from known proteins; the only domain detected showed distant homology to the WSD motif. We show that both ICOP1 and ICOP2 interact with the chromatin remodeler ISWI1. Upon ICOP knockdown, changes in alternative IES excision boundaries and nucleosome densities are similar to those observed for ISWI1 knockdown. We thus propose that a complex comprising ISWI1 and either or both ICOP1 and ICOP2 are needed for chromatin remodeling and accurate DNA elimination in Paramecium.