Effect Of CHB101 Knockdown,A Swi3d Chromatin Remodeler Gene,on Global DNA Methylation In Maize

The gene expression patterns is heritably maintained by the mediation of epigenetic modifications, which include methylation of cytosine bases in DNA, covalent histone modifications, the incorporation of histone variants, the non-covalent remodeling of nucleosomes by ATP-dependent nucleosome remodeling enzymes and non-coding RNAs. Chromatin remodeling is fundamental to gene regulation in eukaryotes and at least two major protein complexes are involved in switching chromatin from "active" to "repress" states and vice versa: the ATP-dependent nucleosome remodeling complex (e.g. the SWITCH2 [SWI2]/SUCROSE NON-FERMENTING2 [SNF2] complex) and histone-modifying complexes (e.g. the histone deacetylase complex[HDAC]).SWI/SNF (SWItch/Sucrose Non-Fermentable) is a group of proteins that associates with nucleosome remodeling and is found in both eukaryotes and prokaryotes. It possesses DNA-stimulated ATPase activity and can destabilize histone-DNA interactions in reconstituted nucleosomes in an ATP-dependent manner. The SWI/SNF complexes are composed of several proteins produced by SWI and SNF genes. It has been found that the SWI/SNF complexes are capable of altering the position of nucleosomes along DNA, thus can promote gene expression. The research about the function of SWI/SNF complex in plants has just begun, compared to that in animals and yeast where it is relatively well documented. To investigate the possible role of maize (Zea mays L.) CHB101 (a homolog of AtSWI3D) in DNA methylation, we used methylation-sensitive amplified polymorphism (MSAP) to compare the methylation at 5'-CCGG-3'sites between transgenic maize (CHB101 knock-down) and segregated non-transgenic maize siblings. Our results showed that:CHB101down-regulated transgenic plants have obvious higher hypermethylation frequency (P<0.001) and lower hypomethylation frequency (P<0.001) in both CG and CHG contexts compared to their segregated non-transgenic sibling plants, although the overall methylation levels between transgenic and non-transgenic plants were not statistically different (P>0.05). The sequencing of 42 differentially methylated fragments showed that they were mainly located in gene body and repetitive elements, but seldom found in gene promoter regions. Bisulfite sequencing of 2 randomly chosen genes confirmed that both hypermethylation and hypomethylation occurred in the transgenic plants depending on the cytosine sites, but the overall methylation levels between transgenic and non-transgenic plants were not significantly different (p>0.05). These results suggested that maize CHB101gene might be involved in the control of CG and CHG methylation process in a cytosine site-specific manner, and therefore provided new prospect for understanding the functions of SWI/SNF complexes in higher plant.