DELETIONS WITHIN THE DNA RECOGNITION SUBUNIT OF M.ECOR124I THAT IDENTIFY A REGION INVOLVED IN PROTEIN-PROTEIN INTERACTIONS BETWEEN HSDS ANDHSDM

The DNA recognition subunit (HsdS) of type I restriction endonucleases can be divided into domains by means of amino acid identity between s ubunits from the same family. It has been proposed that DNA-protein in teractions occur within the variable domains of the subunit and that p rotein-protein interactions involve the conserved domains. We have con structed a number of deletion mutants of HsdS that have allowed us to investigate protein-protein interactions. Using a combination of a ''c ompetitive'' complementation assay and the ability of HsdM to ''solubi lize'' HsdS, we have defined a region within the central conserved dom ain of HsdS that is responsible for HsdS-HsdM interaction. Computer an alysis of amino acid identity between the N-terminal half and the C-te rminal half of HsdS identifies a region (repeated in both conserved do mains), one copy of which overlaps the region we have identified as es sential for HsdS-HsdM interactions, which may be responsible for such protein-protein interactions.