![]() Whether DNA methylation is the first step in the process or a later stabilization of gene silencing is still unclear. DNA regions present as dsRNA become specifically and densely methylated at all cytosines ( 10, 20, 21) and repeat induced methylation usually also coincides with the repeated regions ( 22). Methylation patterns can be exchanged between homologous DNAs in a recombination-like process ( 19). Alternatively, DNA–DNA interactions between homologous regions may be involved ( 15–18). It can be induced by the presence of double-stranded (ds) RNA, which covers transcription control regions ( 10–14). Transcriptional gene silencing in plants is often associated with DNA methylation ( 7, 8) and also includes chromatin-remodeling steps ( 6, 9). It is still unclear how many mechanisms exist to single-out specific genes or chromosome regions for packaging into repressive chromatin and what contribution the individual modifications of chromatin have on the regulation of gene expression. Alterations of chromatin structure require a complex chromatin-remodeling machinery ( 1, 6). The silent state can be developmentally controlled or completely stable and even inheritable ( 4, 5). Silent chromatin is often associated with the presence of deacetylated and specifically methylated histones ( 1, 2) and with cytosine-methylated DNA ( 3). ![]() Chromatin structure can inactivate genes or even chromosomes despite the presence of activating transcription factors. The results indicate that different stages are involved in DNA methylation-correlated gene inactivation, and that at least one of them may involve the attraction of a sequence and methylation-specific DNA-binding protein.Īctivity of a gene in a chromosomal context is determined by the interaction of general and specific transcription factors with a gene-specific set of cis-acting sequences in the context of chromatin structure. In further generations of homozygous offspring the methylation spread into the transcribed region and gene activity was completely repressed also in nonvascular cells. Instead, promoter methylation enabled the alternative binding of a protein with specificity for sequence and methylation. Methylation per se did not inhibit the binding to the promoter region of protein factors which also bound to the unmethylated sequence. The gene activity could be reestablished by treatment with 5-azacytidine. This expression pattern was similar to that of a promoter with a deletion of a vascular bundle expression element. Methylation was observed only in the homozygous offspring and was initially restricted to the promoter region and accompanied by loss of expression in the vascular bundle tissue only. In a transgenic rice line, a β-glucuronidase reporter gene under the control of the rice tungro bacilliform virus promoter became gradually methylated, and gene activity was lost concomitantly.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |