Evidence for a repeating domain in type I restriction enzymes.

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Abstract
The primary structures of the recognition subunit (hsdS) in type I restriction enzymes from three isolates of Escherichia coli were compared and aligned by use of amino acid physical properties. A repeating domain was found in each of the subunits suggesting a pseudo‐dimeric structure. Secondary structure predictions delineated two helical regions in each domain which suggested that the recognition subunits may act in a fashion similar to that proposed for repressor and activator molecules; namely, interaction with double‐stranded DNA through helices and in two successive major grooves on the same DNA side. One helical motif could provide the specific recognition site and the other, the restriction site.