Generation of new DNA binding specificity by truncation of the type IC EcoDXXI hsdS gene

The hsdS subunit of a type IC restriction-modification enzyme is responsible for the enzyme's DNA binding specificity. Type I recognition sites are characterized by two defined half-sites separated by a non-specific spacer of defined length. The hsdS subunit contains two independent DNA binding domains, each targeted towards one DNA half-site. We have shown previously that the 5' half of hsdS can code for a functional substitute of the full-length hsdS. Here we demonstrate that the 3' half of the gene, when fused to the appropriate transcriptional and translational start signals, also codes for a peptide which imparts DNA binding specificity to the enzyme. About half the natural hsdS size, the mutant peptide contains a single DNA recognition domain flanked by one copy of each internal repeat found in the full-length hsdS. Deletion of either repeat sequence results in loss of activity. Like the 5' hsdS mutant, the 3' mutant recognizes an interrupted palindrome, GAAYN(5)RTTC, suggesting that two truncated subunits participate in DNA recognition. Co-expression of the 5' hsdS mutant and the 3' hsdS mutant along with hsdM regenerates the wild-type methylation specificity. Thus, there is a free assortment of subunits in the cell.