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Simplicity of Histone Code Revealed
By Proceedings of the National Academy of Sciences,
Mar 30, 2005 - 6:44:38 AM
Histones, which package DNA in eukaryotes, play an important role in gene regulation. According to the histone code hypothesis, covalent posttranslational modification of histone tails, in this case, acetylation, influences gene regulation by altering transcriptional output.
Michael Dion et al. tested this hypothesis by creating mutant yeast strains in which the four lysines (K) in the histone H4 tail of budding yeast, lysines 5, 8, 12, and 16, were replaced with arginine (R) in all 16 possible combinations. The replacement of lysine with arginine mimics the unacetylated lysine. The team analyzed the gene expression with DNA microarrays and showed that only the K16R substitution changed gene expression of a unique set of genes. Of the 125 genes whose expression differed, 67 showed a >2-fold change.
The authors suggest that this result is consistent with a transcription mechanism involving a K16-binding protein. Mutations in the other three lysines did not uniquely affect gene expression. Instead, the total number of these K-to-R mutations caused incremental genome-wide shifts in expression. The authors conclude that the four H4 lysines produce a total of 8 transcription states rather than the possible 16, suggesting that the histone code is simpler than expected.
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