A mutation that converts serine340 of the HsdSK polypeptide to phenylalanine and its effects on restriction and modification in Escherichia coli K-12

A hybrid hsdS gene, encoding the HsdSts + d polypeptide, was constructed by joining the proximal region of the wild-type (wt) hsdS sequence with the distal region of the hsdSts + d sequence, at the hsdS BglII site. The hybrid hsdS-Sts + d gene exerts a trans-dominant effect on restriction and modification, which points to the location of the temperature-sensitive (ts) trans-dominant (+ d) mutation in the gene hsdSts + d distal region. Sequencing of the region downstream from the HindIII target in the Escherichia coli K-12 hsdSts + d mutant was carried out. It is identical to the wt hsdS sequence (GenBank/EMBL accession number ECHSDK LV00288), except for a single base-pair transition C1245---T. The results obtained support the idea that the trans-dominant effect of the ts mutation described earlier is related to the single base-pair transition in the nonhomologous region of the hsdSts + d sequence.