The program of gene expression during development of plants and animals is controlled by epigenetic mechanisms . This control can be accomplished by methylation / demethylation of DNA or methylation / demethylation of specific lysines in histones H3 and H4 . Polycomb group proteins ( PcG ) is responsible for maintaining repressed genes and the group of proteins tritorax ( trxG ) genes maintained in the active state . Methylation of lysine 9 and 27 in histone H3 lysine 20 methylation and histone H4 results in gene silencing . Histone methyltransferase ( HMTazy ) are appurtenant to the PcG proteins . In transcriptionally active chromatin occurs and methylation of lysine 4 of histone H3 36 and the fifth and eighth lysines in histone H4 . These HMTazy and deacetylase are present among proteins trxG.Najwcześniej PcG proteins have been discovered uDrosophila melanogaster as factors necessary for proper control of homeotic gene expression . TrxG Proteins provide the body with the activity of homeotic genes necessary for the development of the individual segments . PcG complexes and trxG represent a general mechanism conservative in nature - from insects to mammals and plants. Both in the animal kingdom and the plant PcG proteins and trxG act on the same target genes , Hox genes mainly . Knowledge about the role of PcG and trxG in plants is mainly supported by studies carried out on the model species - Arabidopsis thaliana . Arabidopsiszawiera PcG number of homologues of these proteins present in Drosophila and mammals (see tab. 1 ) and homologs animal protein called LHP1 HP1 ( HP1 -like ) . In animals, PcG action is based on a multiprotein complexes PRC1 and PRC2 ( Polycomb repressive called Complexes ) . PRC2 contains HMTazę for H3K27me3 . In plants, there are complexes similar to PRC2 ( PRC2 -like ) . It is suggested that functions in plants like LHP1 PRC1 , that participates in the development of Arabidopsis chromatyny.U remodeling generative reproduction system regulated by PRC2 complexes like , that control the development of the gametophyte female ( embryo sac ) , endosperm and germ . There is a view according to which the two types of complexes similar to PRC2 regulate different target genes , but the main group of target genes for PcG genes are homeotyczne.Tritorax1 Arabidopsis flower ( ATX1 ) is a close homolog of TRX protein in mice. Domain SET ATX probably has weak activity HMTazy for H3K4me . The activity of histone H3K4me3 HMTaz for and H3K36me3 show other trxG protein complex . TrxG target genes of the Hox genes are floral and FLC gene , involved in the process of vernalization (see Tab. 3). DNA methylation and histone modifications cause gene silencing of genes responsible for the expression , depending on the parental origin (so-called parent -of -origin effect) . Among the eleven genes examined uArabidopsis thaliana and Zea mays , only two were silenced by the PcG proteins - one maternal and one paternal ; in five genes deactivation of paternal alleles was caused by DNA methylation and the silencing mechanism other three paternal alleles is unknown (see Tab. 3). Although the study of systems PcG and trxG plants over the past decade have been very fruitful , like many animal material obtained is not resolved the fundamental issue , namely how PcG molecular mechanism prevents transcription. It's not yet clear whether the repressive methylation of histone H3 and H4 core the only function of the PcG proteins or for silencing the expression of genes responsible are modifications of the basic mechanisms governing the transcription.