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I show that the recognition sequences of Type II restriction systems are correlated with the G + C content of the host bacterial DNA. Almost all restriction systems with G + C rich tetranucleotide recognition sequences are found in species with A + T rich genomes, whereas G + C rich hexanucleotide and octanucleotide recognition sequences are found almost exclusively in species with G + C rich genomes. Most hexanucleotide recognition sequences found in species with A + T rich genomes are A + T rich. This distribution eliminates a substantial proportion of the potential variance in the frequency of restriction recognition sequences in the host genomes. As a consequence, almost all restriction recognition sequences, including those eight base pairs in length (Not I and Sfi I), are predicted to occur with a frequency ranging from once every 300 to once every 5,000 base pairs in the host genome. Since the G + C content of bacteriophage DNA and of the host genome are also correlated, the data presented is evidence that most Type II "restriction systems" are indeed involved in phage restriction.

Recognition sequences of Type II restriction systems are constrained by the G+C content of host genomes