Tissue specificity and clustering of methylated cystosines in bovine satellite I DNA.

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Abstract
The positions of all 5-methylcytosine (mC) residues in bovine satellite I DNA were determined by sequence analysis of native purified satellite I DNA from 3 bovine tissues as well as from cloned DNA. The EcoRI cleavage units from thymus and liver were found to contain 1402 residues; that from brain contained 1401 residues. Satellite I DNA from thymus contained a total of 5.0% mC, whereas that from liver and brain contained 4.4% and 2.6% mC, respectively. The extent of methylation of this DNA is tissue-specific. So is the location. In each tissue, the location of mC is nonrandom, consisting of 3 clusters of heavily methylation within each cluster is tissue-specific. The mC are located entirely in C-G doublets and primarily in palindromic sequences. C-C-G-G sequences (10 methylatable sites) are almost completely methylated in all tissues examined, but T-C-G-A sequences (16 methylatable sites) are methylated to different extents in each tissue. Neither the tissue specificity of methylation nor the clustering pattern is detectable by examining only C-C-G-G sites, emphasizing the importance of total sequence determination for genomic DNA in studies of methylation. The clustering pattern, which is preserved despite a 2-fold difference in mC content between brain and thymus, may indicate a role for DNA methylation in chromatin structure.