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A novel DNA sulfur modification was discovered [1],and it has been identified as a phosphorothioate modification which is the first known physiological modification of DNAs backbone [2-4].In vitro the phosphorothioated DNA is degraded during normal and pulsed-field gel electrophoresis,which was named Dud phenotype (DNA degradation),but in vivo the function of the phosphorothioate modification of DNA remained obscure.Here.we describe the cloning and characterization of the dpt gene cluster for an unusual phosphorothioation-dependent restriction modification (dptR-M) system,dptB-H,from Salmonella enterica serovar Cerro 87.The entire gene cluster (dpt) involved in this R-M system was localized on a 22.5kb DNA fragment and expressed heterologously in Escherichia.coli DH5α This system,with two different convergent transcripts.