[The relation of enzymatic methylation of inverted DNA repeats to transcription in mouse P815 mastocytoma cells (author's transl)].

The isolation of transcribed DNA sequences of P815 cells and the partial characterization with respect to their sequence composition and relative rates of enzymatic DNA methylation are reported in this paper. Transcribed regions were purified by affinity chromatography using immobilized heterogenous nuclear RNA of P815 cells. About 10% of total genome was found in this fraction. Reassociation analyses showed differences in sequence composition of transcribed versus non-transcribed DNA fractions. The relative proportion of inverted repeats was doubled in the transcribed fraction whereas ordinary highly repetitive sequences comprising mainly of satellite DNA were found almost exclusively in the non-transcribed regions of the P815 genome. About 70% of transcribed portions corresponds to unique and intermediary DNA sequences. After labelling of cells with L-[Methyl-3H]methionine and [14C]deoxycytidine relative rates of enzymatic DNA methylation were computed for different kinetic components of transcribed and non-transcribed portions of P815 genome. No difference was found except in inverted repeats. In transcribed DNA the relative rate of enzymatic DNA methylation was only about 40% of that of the non-transcribed ones. We have quantitated this hypomethyltion and found that there is, in average, about one 5-methylcytosine residue in 100 nucleotides of transcribed inverted repeats, compared to about 2.5 5-methylcytosines in non-transcribed fractions. In view of these data we propose that the enzymatic methylation of inverted DNA repeats negatively controls the transcriptional process in a given genomic region.