The Arabidopsis PEAT Complex Is Involved In Histone Deacetylation And Heterochromatin Transcription Silencing

In eukaryotes,pericentromeric heterochromatin regions contain a large number of transposable elements(TEs),which are typically transcriptional silenced to ensure genome stability.DNA methylation and histone deacetylation play important roles in maintaining transcriptional silencing.Heterochromatin regions produce a large number of 24-nt small interfering RNAs(siRNAs),which are required for establishing and maintaining DNA methylation through RNA-directed DNA methylation pathway(RdDM)in plants.However,little is known how DNA methylation,histone deacetylation and the production of siRNAs intercoordinate to mediate the transcriptional silencing.In order to identify unknown regulators that are involved in heterochromatin silencing,we conducted a reverse genetic screen.We screened a collection of SALK T-DNA insertion mutants for whose expression of the solo LTR(solo long terminal repeat)was elevated.Through the screen,we identified a new regulator required for TE transcriptional silencing.The regulator is similar to the conserved EPC1 subunit of the NuA4 histone acetylase complex in eukaryotes,named EPCR1 and EPCR2(EPC1-related protein 1 and 2).EPL1a and EPL1b are two orthologs of EPC1 in Arabidopsis thaliana,acting as subunits of the NuA4 complex.EPCR1 and EPCR2 are however not subunits of the NuA4 complex.By affinity purification in combination with mass spectrometry,we found that ECPR1 and EPCR2(EPCR1/2)interact with PWWP1/2/3,ARID2/3/4 and TRB1/2,forming multiple functionally redundant protein complexes named PEAT(PWWPs-EPCRs-ARIDs-TRBs).In vitro protein binding assay indicated that,in the PEAT complexes,PWWPs are able to interact directly with three other proteins and act as core components of the complexes.Moreover,affinity purification in combination with mass spectrometry demonstratedrated that the PEAT complexes interact not only with the histone acetyltransferases HAM 1/2 but also with the histone deacetylase HDA6.In PEAT mutants,the expression of TEs and other DNA repeats from heterochromatin regions were increased.Meanwhile,H4 lysine 5 acetylation(H4K5ac)levels in these regions were increased,accompany by decreased heterochromatin compaction.Moreover,Pol IV dependent 24-nt siRNAs and DNA methylation were increased in the heterochromatin regions.Taken together,our study suggests that the PEAT complexes mediate histone deacetylation and heterochromatin condensation and thereby facilitate heterochromatin silencing.Moreover,the complexes inhibit the excessive accumulation of siRNAs in heterochromatin regions and thereby affect RNA-directed DNA methylation.