The usual source of restriction endonucleases used in gene cloning is 

Bacterial species are the major source of commercial restriction enzymes. These enzymes serve to defend the bacterial cells from invasion by foreign DNA, such as nucleic acid sequences used by viruses to replicate themselves inside a host cell. Basically, the enzyme will chop DNA into much smaller pieces which pose little danger to the cell. The enzymes are named for the species and strain of bacteria that produces it. For instance, the first restriction enzyme extracted from Escherichia coli strain RY13 is called EcoRI, and the fifth enzyme extracted from the same species is called EcoRV.