Alteration of Developmental Program in Paramecium by Treatment with Trichostatin A: A Possible Involvement of Histone Modification

In the ciliate Paramecium caudatum, the somatic macronucleus for the next generation differentiates from a zygotic germinal micronucleus during conjugation. Subsequently, progeny are programmed to enter a sexually “immature” period, during which cells do not mate. This programming occurs at a critical period during the fifth cell cycle after conjugation. Here, we show that treatment with trichostatin A, a histone deacetylase inhibitor, leads to a partial recovery of mating reactivity in immature cells, suggesting that histone acetylation can affect the developmental program. In addition, immunostaining demonstrated that Lys 4 methylation of histone H3 was absent from or present at an undetectable level in the nascent macronucleus during the first cell cycle after conjugation. The methylation level gradually increased in proportion to de novo DNA synthesis, until the new macronucleus reached the maximum level of methylation, concomitant with its full maturation. A link between gene expression and Lys 4 methylation is indirectly supported by the observation that a transcriptionally active gene was enriched by chromatin immunoprecipitation with an antibody directed against methyl-Lys 4 H3, whereas a silent gene was not. These results provide evidence that histone modification plays a key role in the regulation of gene expression and the developmental programming that determine sexual immaturity after conjugation.