An investigation into the significance of histone methylation in various mouse tissue
Histone lysine methylation is a post synthetic modification that occurs on the N-terminal tails of histones H3 and H4. This modification occurs at very specific residues, which have been highly conserved throughout evolution. It has been postulated that histone methylation may be involved in the pre-mitotic condensation of chromatin. Others have speculated that it may be involved in transcription. This modification seems to be a general feature of chromatin and occurs in all eukaryotic organisms ranging from mammals to yeast. The functional significance of histone methylation and of proteins in general is still at this stage not very well understood in the first part of this project the levels of histone lysine methylation in various tissues were investigated in order to establish a correlation between the methyl content and cell division, age and differentiation. It was found that the levels of histone lysine methylation were low in the two tissue culture lines that were investigated correlating with the rapid growth rate of these cells. An increase in the histone lysine methylation content was observed when these cells were induced to differentiate. The highest levels of histone lysine methylation were found in brain tissue. Methylation levels in the histones isolated from the mouse and bovine brain tissue are very similar. It was concluded from these results that histone lysine methylation may be linked to the cessation of replication and differentiation of cells. Previous studies on the rate of methylation in rat brain (Lee and Duerre, 1974) have shown that the rate of histone methylation decreases with age of the tissue. It was therefore decided to investigate the rate of histone methylation in nuclei isolated from mouse erythroleukemia (MEL) cells at different stages of differentiation. These cells can be induced to differentiate upon which they cease to replicate and mature into cells that are analogous to red blood cells. The rate of incorporation of radioactive methyl groups into the histones was determined in nuclei isolated from rapidly growing cells and cells that had been induced to differentiate for one and four days. It was established that only histones H3 and H4 are methylated at significant levels. The highest rate of histone methylation occurred in histone H3, and to a much lower degree, H4. This rate was maintained for the first day and decreased significantly after the fourth day after induction. Even after the fourth day after induction, the rate of histone H3 methylation remained significantly higher than that of the other histones. It was concluded that histone methylation proceeds for a substantial amount of time after all DNA replication has ceased. In the final part of this project the effect of the level of histone methylation on the stability of core particles, chromatin and nuclei was investigated. The core particles and chromatin, isolated from tissues with different levels of histone lysine methylation, were 'melted' using UV thermal denaturation. No differences were found. It was therefore concluded that histone lysine methylation did not increase the stability of the DNA of core particles or of soluble chromatin from these samples. Differential scanning calorimetry was used to study the denaturation of nuclei isolated from mouse brain and MEL cells that have high and low levels of histone lysine methylation respectively. The "melting profiles" of the two types of nuclei seem to indicate that structural differences exist. Due to the complex structure of a nucleus it is uncertain whether these differences could be ascribed to the differing levels of histone lysine methylation.
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Department of Molecular and Cell Biology