Elevated transcriptional complexity and decrease in enzymatic DNA methylation in cells treated with L-ethionine.

This paper analyzes the extent of enzymatic methylation of DNA in P815 cells cultured for several cell cycles in the presence of 1 mM L-ethionine by use of the bacterial restriction enzymes HpaII, MspI, HhaI, HaeII, and AvaI. The results obtained suggest that the extent of methylation of DNA is decreased during L-ethionine treatment and that the degree of this inhibition varies among different methylation sites. The nucleotide sequence complexity of nuclear RNA in P815 cells was analyzed by excess hybridization with tracer amounts of nick-translated nonrepetitive P815 [3H]DNA sequences. About 6.2% of the nonrepetitive sequences form hybrids with nuclear RNA of controls cells, whereas 8.8% are complementary to nuclear RNA from L-ethionine-treated cells. Thus, the decrease in enzymatic DNA methylation correlates with the presence of new transcriptional products suggesting that this modification process inversely correlates with the initiation of transcription.