title: In Z-DNA the sequence G-C-G-C is neither methylated by Hha I methyltransferase nor cleaved by Hha I restriction endonuclease.
abstract: Plasmids carrying 24- or 32-base-pair inserts of alternating (dG-dC) residues were used to analyze the level of methylation of the G-C-G-C sites by Hha I DNA methyltransferase and their cleavage by Hha I endonuclease in the B-DNA or Z-DNA conformation. In supercoiled plasmids in which the inserts formed Z-DNA, the extent of methylation at the insert G-C-G-C sites was dramatically lower than the level of methylation at the G-C-G-C sites located outside the insert in the same plasmid. Similarly, cleavage by Hha I endonuclease was sharply lowered when the insert was in the Z-DNA form. In the relaxed plasmid, all its G-C-G-C sites were methylated to the same extent and the unmethylated sites were readily cleaved. After treatment with the methylase, the supercoiled plasmid was linearized and then digested with Hha I restriction endonuclease. This exposed unmethylated G-C-G-C sites from the insert that had been protected against cleavage in the Z conformation. A chemical reaction was used to study the distribution of the unmethylated cytosine residues. No accumulation of unmethylated cytosine residues was found anywhere along the entire 32-base-pair insert, which is consistent with a cooperative B-Z transition.
mesh_terms: D001483:Base Sequence; D004247:DNA / Q000378:metabolism; D004248:DNA (Cytosine-5-)-Methyltransferases / Q000378:metabolism; D004262:DNA Restriction Enzymes / Q000378:metabolism; D004278:DNA, Superhelical / Q000235:genetics; D015257:DNA-Cytosine Methylases; D015252:Deoxyribonucleases, Type II Site-Specific; D008745:Methylation; D009690:Nucleic Acid Conformation; D010957:Plasmids; D013379:Substrate Specificity
chemical_list: D004278:DNA, Superhelical; D004247:DNA; C037664:DNA modification methylase HhaI; D015257:DNA-Cytosine Methylases; D004248:DNA (Cytosine-5-)-Methyltransferases; D004262:DNA Restriction Enzymes; D015252:Deoxyribonucleases, Type II Site-Specific; C037130:GCGC-specific type II deoxyribonucleases