Cytosine Methylation by the Sua I Restriction-Modification System: Implications for Genetic Fidelity in a Hyperthermophilic Archaeon

Abstract

ABSTRACT 5-Methylcytosine in chromosomal DNA represents a potential source of frequent spontaneous mutation for hyperthermophiles. To determine the relevance of this threat for the archaeon Sulfolobus acidocaldarius , the mode of GGCC methylation by its restriction-modification system, Sua I, was investigated. Distinct isoschizomers of the Sua I endonuclease were used to probe the methylation state of GGCC in native S. acidocaldarius DNA. In addition, the methylation sensitivity of the Sua I endonuclease was determined with synthetic oligonucleotide substrates and modified natural DNAs. The results show that the Sua I system uses N 4 methylation to block cleavage of its recognition site, thereby avoiding the creation of G · T mismatches by spontaneous deamination at extremely high temperature.