A Developmentally Regulated Gene, ASI2, Is Required for Endocycling in the Macronuclear Anlagen of Tetrahymena
Format of Original
American Society for Microbiology
Ciliated protozoa contain two types of nuclei, germ line micronuclei(Mic) and transcriptionally active macronuclei (Mac). Duringsexual reproduction, the parental Mac degenerates and a newMac develops from a mitotic product of the zygotic Mic. Macronucleardevelopment involves extensive endoreplication of the genome.The present study shows that endoreplication of macronuclearDNA in Tetrahymena is an example of endocyling, a variant ofthe mitotic cycle with alternating S and G phases in the absenceof cell division. Thus, endocycling is conserved from ciliatesto multicellular organisms. The gene ASI2 in Tetrahymena thermophilaencodes a putative signal transduction receptor. ASI2 is nonessentialfor vegetative growth, but it is upregulated during developmentof the new Mac. Cells that lack ASI2 in the developing Mac anlagenare arrested in endoreplication of the DNA and die. This studyshows that ASI2 is also transcribed in the parental Mac earlyin conjugation and that transcription of ASI2 in the parentalMac supports endoreplication of the DNA during early stagesof development of the Mac anlagen. Other molecular events inMac anlage development, including developmentally regulatedDNA rearrangement, occur normally in matings between ASI2 knockouts,suggesting that ASI2 specifically regulates endocycling in Tetrahymena.