In vivo and in vitro experiments carried out on L929 mouse fibroblasts suggested 
that the poly(ADP-ribosyl) ation process acts somehow as a protecting agent 
against full methylation of CpG dinucleotides in genomic DNA. Since CpG islands, 
which are found almost exclusively at the 5'-end of housekeeping genes, are rich 
in CpG dinucleotides, which are the target of mammalian DNA methyltransferase, 
we examined the possibility that the poly(ADP-ribosyl)ation reaction is involved 
in maintaining the unmethylated state of these DNA sequences. Experiments were 
conducted by two different strategies, using either methylation-dependent 
restriction enzymes on purified genomic DNA or a sequence-dependent restriction 
enzyme on an aliquot of the same DNA, previously modified by a bisulfite 
reaction. With the methylation-dependent restriction enzymes, it was observed 
that the "HpaII tiny fragments" greatly decreased when the cells were 
preincubated with 3-aminobenzamide, a well known inhibitor of poly(ADP-ribose) 
polymerase. The other experimental approach allowed us to prove that, as a 
consequence of the inhibition of the poly(ADP-ribosyl)ation process, an 
anomalous methylation pattern could be evidenced in the CpG island of the 
promoter fragment of the Htf9 gene, amplified from DNA obtained from fibroblasts 
preincubated with 3-aminobenzamide. These data confirm the hypothesis that, at 
least for the Htf9 promoter region, an active poly(ADP-ribosyl)ation protects 
the unmethylated state of the CpG island.