Bacterial viruses also called bacteriophages, constantly infect bacteria. As a result, bacteria produce restriction enzymes that protect them from viral DNA and restrict infection. Three types of restriction enzymes exist and each type seeks out a single DNA sequence and precisely cuts it in one place. According to Promega Corporation, there are about 3,000 very selective restriction enzymes. Current DNA technology, including cloning and typing, rely totally on restriction enzymes, most of which are derived from bacteria, but some come from viruses and other organisms.
Type 1
Type 1 restriction enzymes are common and recognize specific DNA sequences but make their cut at sites approximately 1,000 base pairs away from the recognition site. Examples include EcoB and EcoK derived from E. coli bacteria.
Type 2
Type 2 restriction enzymes recognize and cut within the recognition site. Examples include HindII and HindIII derived from H. influenzae bacteria. DNA analysis and gene cloning relies on this type of restriction enzyme.
Type 3
Type 3 restriction enzymes recognize specific sequences but make their cut at a specific location usually within about 25 base pairs of the recognition site. These enzymes have no commercial application. An example is HgaI derived from Haemophilus gallinarum bacteria.
