2 Weircleases (DNases) and then to focus on the techniques in which they are
used. The enzymes that molecular biologists use as tools are dealt with
in separate chapters in this volume.
1.1. Nomenclature
Anyone who has tried the isolation of a DNase enzyme will know
that the presence of multiple types of nuclease activity makes this
process fraught with difficulty. In this section, consideration will be
given to the properties of the DNase enzymes with a view to under-
standing their nomenclature, which for the most part is somewhat
confusing (Table 1).
Nucleases, although a large group in themselves, are part of a larger
group of enzymes, the phosphodiesterases, which are able to catalyze
the cleavage of phosphate-ester bonds. Schmidt and Laskowsi (2) iden-
tified three types of nuclease enzymatic activity: DNases, ribonucle-
ases (RNases), and exonucleases. On this definition, it is apparent that
so-called DNases and RNases cleave their substrates endonucleolytically,
i.e., at internal sites, and that this activity is distinct and separable from
any exonuclease activity. In practical terms, this definition holds true
in that an endo-DNase will not digest DNA molecules to completion,
i.e., to nucleotide monomers; only when exonuclease activity is present
will the digestion of DNA go to completion.
A second confusing element in the nomenclature of nucleases, and
DNases in particular, is the presence of single-stranded DNases, e.g.,
mung bean nuclease and nuclease S 1 fromAspergillus. These enzymes,
although having high specificity for single-stranded DNA molecules,
will, at high concentrations and in preparations not purified to homoge-
neity, digest native (double-stranded) DNA molecules albeit at reduced
rates. For an example of this, Weir and Bryant (3) have isolated a nuclear-
located DNase from the embryo axes of pea that has a low, but mea-
surable activity on native DNA but rapidly catalyzes the hydrolysis of
heat-denatured DNA. It is not known so far whether these two activi-
ties are separable, but evidence from similar enzymes suggests that these
activities are part of the same protein molecule. DNases then, tend to
be classified as to "what they do best"; e.g., the DNase of Weir and Bryant
would be called a single-strand specific endo-DNase. In the following
discussion, the examples are from the DNase class of nucleases, how-
ever all the points considered can be equally applied to the RNases.