Deletions within the DNA recognition subunit of M.EcoR124I that identify a region involved in protein-protein interactions between HsdS and HsdM.

The DNA recognition subunit (HsdS) of type I restriction endonucleases can be divided into domains by means of amino acid identity between subunits from the same family. It has been proposed that DNA-protein interactions occur within the variable domains of the subunit and that protein-protein interactions involve the conserved domains. We have constructed a number of deletion mutants of HsdS that have allowed us to investigate protein-protein interactions. Using a combination of a "competitive" complementation assay and the ability of HsdM to "solubilize" HsdS, we have defined a region within the central conserved domain of HsdS that is responsible for HsdS-HsdM interaction. Computer analysis of amino acid identity between the N-terminal half and the C-terminal half of HsdS identifies a region (repeated in both conserved domains), one copy of which overlaps the region we have identified as essential for HsdS-HsdM interactions, which may be responsible for such protein-protein interactions.