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Identifiers and Description
Gene Model Identifier
PDD3 (Programmed DNA Degradation )
PreTt17969 | 239.m00048 | 3802.m00189
Programmed DNA degradation 3 protein; developmentally regulated chromodomain protein; localizes to DNA elimination structures during programmed DNA elimination; binds methylated Lys-9 of histone H3
A proposed model for the mechanism of programmed DNA elimination in Tetrahymena is based on the timing of expression, cellular distribution, mutant phenotypes, and predicted functions of the protein and RNA components involved. In this model, both strands of the micronuclear genome (or perhaps only the portions containing internal eliminated sequences) are transcribed early in conjugation to produce large non-genic, double-stranded RNAs. This transcription is likely performed by RNA Polymerase II, based on the localization of its subunit Rpb3p to the micronucleus during this time. These transcripts pass to the cytoplasm where they are processed into short (~28 nucleotide) scan RNAs (scnRNA) by the dicer-like protein Dcl1p, similar to the production of the small inhibitory RNAs (siRNA) central to the RNA interference (RNAi) pathway of other eukaryotes. The scnRNAs complex with Twi1p, a member of the PPD (PAZ and Piwi Domain) protein family, whose members are commonly involved in RNAi and related processes. The scnRNA/Twi1p complexes enter the old macronucleus, where scnRNAs homologous to DNA sequences found there are degraded. The remaining scnRNAs, comprised of micronuclear-restricted sequences, are transferred to the developing macronucleus. There, histone H3 proteins (Hht1p, Hht2p) that are bound to sections of the genome sharing identity to the scnRNAs are methylated on lysine-9. This modification, which is often associated with the formation of heterochromatin, is recognized by one or more of the chromodomains belonging to Pdd1p and Pdd3p. Regions of DNA associated with these modified histones are eliminated from the developing macronuclear genome.
Ref:12297044: Taverna SD, Coyne RS, Allis CD (2002) Methylation of histone h3 at lysine 9 targets programmed DNA elimination in tetrahymena. Cell 110(6):701-11
Ref:10805754: Nikiforov MA, Gorovsky MA, Allis CD (2000) A novel chromodomain protein, pdd3p, associates with internal eliminated sequences during macronuclear development in Tetrahymena thermophila. Molecular and cellular biology 20(11):4128-34
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