Preferential methylation of regulatory genes in hela cells

Abstract

1. Introduction The genome of a cell is exposed to the action of two classes of DNA-modifying enzymes: (1) a DNA methyl- ase which transforms roughly l/l 5 of its cytosines to 5-methyl-cytosines, while (2) a mere I/ 1000 of these 5-methyl-cytosines are transformed by a DNA deami- nase to minor thymines [l-3]. Transformation from cytosine to 5-methylcytosine can take place in isolated nuclei of sea urchin eggs [4] and HeLa cells [5]. But, at least for HeLa cells, transformation from 5-methyl- cytosine to ‘minor’ thymine does not take place in isolated nuclei [5] and probably is under cytoplasmic control. This was suggested by investigation on syn- chronized HeLa cell suspension in which [’ 4C]methyl- Lmethionine was employed as the sole tracer for both methylation and synthesis of DNA [5 ] : The labelled C-atom of the methyl group of L-methio- nine does not enter the pyrimidine ring [2] : via methy- lation it is transferred to DNA 5-methylcytosine; via Cl -intermediates it enters the purine ring and the methyl group of thymine to participate in the DNA biosynthetic process. The two pathways of DNA methylation and DNA synthesis were then separated from each other during the HeLa cell cycle [5] : in the whole cell, DNA methylation parallels DNA syn- thesis during the S-phase; in the isolated nuclei, DNA methylation proceeds during the S-phase in absence of DNA synthesis. Therefore, completed DNA chains can be methylated (furthermore, Adams [6] has demonstrated that newly synthesized chains are not methylated); their methylation during S might represent a condition of DNA duplication or of gene expression [2,4]. S-CH3 ----+ SAM - DNA (Cytosine + 5-methyl-cytosine 4 Minor thymine) I Methylase Deaminase (CH,), t t H,N-CH-COOH -_---_ ---------__--_ Methylase pathway Nucleus i _ ________ _______----_---_- - ---- 4 J(Oxidation) Biosynthetic pathway - Cytoplasm 4 C 1 -chain + dUMP -+ dTMP + dTTP 4 J- Cytoplasmic + regulator Adenine-----+ dATP Guanine------+ dGTP