A restriction enzyme is a component of a restriction-modification system of a given specificity within an organism. The R-MT system consists of two enzymatic components, a restriction endonuclease, and a modification enzyme sharing similar (or identical) recognition specificity. We would like to propose that the genus-species designation be used as an R-M system name according to the following rules.
(1) The genus and species name of the host organism is identified by the first letter of the genus and the first two letters of the species to form a three-letter abbre- viation in italics. For example: E. coli, Eco and H. influenzae, Hin.
(2) Strain or type identification follows the genus-species abbreviation in non- italicized symbols, e.g. EcoB or EcoK. In cases where the R-M system is genetically specified by a virus or pla,smid the italicized genus-species abbreviation of the host is given and the symbol for the extrachromosomal element follows in non-italics, e.g. EcoPl, EwRI, etc. In occasional cases where it might be necessary to specify the host strain as well as the extrachromosomal element the strain identification symbol may be inserted parenthetically, e.g. Eco(B)Pl.
(3) When a particular host strain has several different R-M systems, these are identified by Roman numerals, thus, the R-M systems from H. injluenzae strain d would be HindI, HindII, HindIII, etc.
Now HindII was -
- extracted from Haemophilus influenzae
- the strain was strain Rd, so d
- and it was the 2nd enzyme to be isolated? Hence II.
So how is it the first Type II RE?
H.O. Smith, K.W. Wilcox, and T.J. Kelley, working at Johns Hopkins University in 1968, isolated and characterized the first restriction nuclease whose functioning depended on a specific DNA nucleotide sequence. (wiki)