The Escherichia coli RglA protein is believed to restrict hydroxymethylated and methylated DNA at a specific base sequence, although its molecular characteristics have never been elucidated. The rglA gene, which encodes the RglA protein, forms a smooth colony when plated with the $Glu^-$ T6 phage and the $Glu^-$ T4 phage. The isolated rglA gene complements various rglA strains, and gives rise to a low transformation frequency of HpaII-methylated pUC13 DNA. The molecular weight of the RglA protein was estimated to be approximately 30 KDa by the use of a T7 RNA polymerase/promoter system. The rglA gene under a T7 promoter is overexpressed by IPTG in an E. coli strain carrying the integrated T7 RNA polymerase gene. This is useful for its purification.