The amino acid side chains of the four histones in the nucleosome are subjected to remarkable variety of post-translation modifications such as phosphorylation, acetylation and methylation. Which one of the following post-translational marks on histone tails is usually associated with transcriptional repression?
Methylation of H3K9
Histone Modifications and Gene Regulation
Histones are proteins that package DNA into structural units called nucleosomes. Four main types of histones (H2A, H2B, H3, and H4) form the core of the nucleosome. The tails of these histones extend outward from the nucleosome core and are subject to various post-translational modifications (PTMs).
These histone modifications play a crucial role in regulating gene expression by influencing chromatin structure and recruiting specific proteins. Different modifications can lead to either transcriptional activation (gene turned ON) or transcriptional repression (gene turned OFF).
Histone Modifications Linked to Repression
The question asks about a specific post-translational modification on histone tails that is usually associated with transcriptional repression. Let's look at the options provided:
- Acetylation: Acetylation of lysine residues on histone tails typically neutralizes their positive charge. This weakens the interaction between histones and the negatively charged DNA, leading to a more open chromatin structure (euchromatin), which is generally associated with transcriptional activation. Acetylation of H3K9 and H4K5 mentioned in the options are examples often linked to activation.
- Methylation: Methylation of lysine or arginine residues can have varied effects depending on the specific residue and the number of methyl groups. For example, methylation on some residues is associated with activation, while on others, it is linked to repression.
- Phosphorylation: Phosphorylation usually adds a negative charge to serine, threonine, or tyrosine residues. This modification is often associated with processes like chromosome condensation during mitosis or meiosis, DNA repair, and also transcriptional activation in response to specific signals. Phosphorylation of H3S10, for instance, is often linked to mitotic chromosome condensation and transcriptional activation.
H3K9 Methylation and Transcriptional Repression
Specifically, methylation at lysine 9 of histone H3 (H3K9) is a well-known epigenetic mark associated with transcriptional repression. When H3K9 is methylated, it often leads to the recruitment of proteins that compact the chromatin structure (forming heterochromatin) and prevent transcription factors from accessing the DNA, thus silencing gene expression.
Comparing this with the other options:
- Acetylation of H3K9 and H4K5 generally promotes a more open chromatin state favorable for transcription.
- Phosphorylation of H3S10 is often linked to transcriptional activation or chromosome condensation during cell division.
Therefore, Methylation of H3K9 is the post-translational modification listed that is typically associated with transcriptional repression.
Important Questions from Genes and chromosomes
Readers of histone modifications include:
In context of DNA methylation, which one of the following statements is FALSE?
The nucleosome is the fundamental subunit of chromatin in eukaryotes. Following statements are made about nucleosome:
A. Generally, a typical nucleosome contains ~ 200bp of DNA and two copies of each histone (H2A, H2B, H3 and H4)
B. 146 bp length of DNA per nucleosome core particle is strictly maintained across the organisms
C. The histone octamers are not conserved during/after replication, however, H32-H42 tetramers are.
D. Variants have been identified for all core histones except histone H3
E. While wrapping around the core histones, the structure of DNA is altered at the middle of the nucleosome core particle and exhibits increased number of base pairs per turn
Which one of the following combination of statements is most appropriate?
Which one of the following statements about Short Interspersed Nuclear Elements (SINEs) is TRUE?
Following statements were made about transposons:
A. Transposons have inverted terminal repeats and their integration generates inverted repeats at the flanks of the target site in the host genome.
B. A composite transposon can transpose as a unit.
C. The transposition event may cause deletions or inversions or move a host sequence to a new location.
D. The transposition event may cause deletions or inversions but cannot move a host sequence to a new location.
E. Replicative transposition proceeds through cointegration.
Which one of the following represents the combination of the correct statements?
