1.1  Restriction EnzymesCitation

When I come to the laboratory of my father, I usually see some plates lying on the tables. These plates contain colonies of bacteria. These colonies remind me of a city with many inhabitants. In each bacterium there is a king. He is very long, but skinny. The king has many servants. These are thick and short, almost like balls. My father calls the king DNA, and the servants enzymes . . . My father has discovered a servant who serves as a pair of scissors. If a foreign king invades a bacterium, this servant can cut him in small fragments, but he does not do any harm to his own king.
—Sylvia (10 years old), daughter of Werner Arber (as quoted in Konforti, 2000)

Swiss microbiologist Werner Arber was one of the recipients of the 1978 Nobel Prize in Physiology or Medicine, an award he earned for his discovery (with Stuart Linn) of restriction enzymes, otherwise known by his daughter Sylvia as "servants with scissors." Arber discovered restriction enzymes while studying a phenomenon known as host-controlled restriction of bacteriophages (also known at the time as host-controlled modification of bacteriophages). Bacteriophages are viral particles that invade bacteria and replicate their own DNA independently of the bacterial chromosomal DNA. Prior to Arber's work, researchers Salvador Luria and Mary Human had shown that various phages were host specific, with each phage surviving and flourishing only in one host bacterial strain and growing poorly in others (Luria & Human, 1952). Those phages that grew poorly were said to be "restricted" by their host. Arber wanted to know why.

Arber proposed that bacterial cells (in this case, E. coli) were able to protect themselves against foreign DNA through some sort of enzymatically catalyzed genetic defense mechanism (Arber & Linn, 1969). Specifically, he theorized that only those bacteriophages that had previously been in contact with the same bacterial strain could successfully infect new host cells, and that the previous exposure somehow modified the phage DNA in a way that protected it from restriction. Phages with unmodified DNA, on the other hand, were immediately broken down by enzymes. This occurred because the host cell enzymes recognized these phages as foreign, cleaving their DNA and restricting their growth. Arber further proposed that there were specific sites in the genome at which restriction activities occurred. Arber and Linn referred to the enzyme responsible for this "endonucleolytic scission" as endonuclease R, a name later changed to EcoB. It didn't take long for other scientists to identify a second restriction enzyme in E. coli: EcoK (Meselson & Yuan, 1968).

Soon after the discovery of EcoB and EcoK, microbiologists Hamilton Smith and Kent Wilcox isolated and characterized the first restriction enzyme from a second bacterial species, Haemophilus influenzae. They also confirmed Arber's hypothesis by demonstrating that HindII, the name eventually given to the enzyme they discovered, degrades foreign phage DNA but not the bacterial host's DNA (Smith & Wilcox, 1970). Smith and his postdoctoral fellow, Thomas Kelly, later identified the actual nucleotide sequence of the specific site where HindII cleaves (known today as a restriction or recognition site), again confirming Arber's hypothesis that restriction enzymes are extremely selective with regard to where they make their cuts (Smith & Kelly, 1970).