Abstract

Histone modification and nucleosome assembly are mainly regulated by various histone-modifying enzymes and chaperones. The roles of histone-modification enzymes have been well analyzed, but the molecular mechanism of histone chaperones in histone modification and nucleosome assembly is incompletely understood. We previously found that the histone chaperone Nrp1 is localized in the micronucleus (MIC) and the macronucleus (MAC) and involved in the chromatin stability and nuclear division ofTetrahymena thermophila. In the present work, we found that truncated C-terminal mutant HA-Nrp1TrCabnormally localizes in the cytoplasm. The truncated-signal-peptide mutants HA-Nrp1TrNLS1and HA-Nrp1TrNLS2are localized in the MIC and MAC. Overexpression of Nrp1TrNLS1inhibited cellular proliferation and disrupted micronuclear mitosis during the vegetative growth stage. During sexual development, Nrp1TrNLS1overexpression led to abnormal bouquet structures and meiosis arrest. Furthermore, Histone H3 was not transported into the nucleus; instead, it formed an abnormal speckled cytoplastic distribution in the Nrp1TrNLS1mutants. The acetylation level of H3K56 in the mutants also decreased, leading to significant changes in the transcription of the genome of the Nrp1TrNLS1mutants. The histone chaperone Nrp1 regulates the H3 nuclear import and acetylation modification of H3K56 and affects chromatin stability and genome transcription inTetrahymena.Keywords:Tetrahymena thermophila;histone chaperoneNRP1;mutation;acetylation of H3K56;genome transcription