Identification of a possible stem-cell-maintenance gene homologue in the unicelluler eukaryote Paramecium caudatum

In macronuclear DNA of the ciliated protozoan Paramecium caudatum, we identified a gene homologous to aubergine/piwi in Drosophila and ZWILLE in Arabidopsis, which have recently been shown to regulate the maintenance of stem cell in animals and plants. The DNA sequence was considered to be actually transcribed because it was also obtained from cDNA of vegetative phase cells by RACE-PCR. The gene named pap (aramecium aubergine/piwi homologue) encodes 781 amino acids and contains three short introns (21bp, 22bp and 23bp) in the ORF. Forty four UAA colons and 8 UAG colons which code for glutamine in Paramecium were found in the ORF. This is the first finding of piwi/ZWILLE homologue in a protozoan. The gene was transcribed actively immediately after induction of conjugation, while the transcription was restricted amount during vegetative phase. The result suggests that the gene is integrated in the developmental process of conjugation.