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Structure-Function Differences Among Type I Restriction EndonucleaseCofactor Requirements of EcoKIS-adenosylmethionine (SAM)SAM has been described as an absolute requirement for specific DNA binding and is thought to act as an allosteric effector (Hadi et al., (1975) J. Biol. Chem. 250, 4159-4164. Two SAM molecules have been found to bind to HsdM and two SAM-binding motifs have been identified. SAM-binding has been shown to alter the conformation of the DNA methyltransferase M2S1. Adenosine triphosphate (ATP)ATP is known to be involved in the determination of the methylation status of the target DNA sequence (Burckhardt et al., (1981) J. Biol. Chem. 256, 4024-4032. Unmodified DNA is the natural substrate for DNA cleavage, but hemi-methylated DNA is the preferred substrate for the methylation activity. Therefore, EcoKI is a maintenance methylase.
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