An endo-DNase with single strand nicking activity from
the bean pathogen, F. solani f. sp. phaseoli
has been previously reported to
induce the expression of defence genes in peas. Culture
filtrates of the potato pathogen, Colletotrichum
coccodes, also contain a single
strand nicking endo-DNase (CCN). This nuclease has a
molecular weight of ∼27 kDa, an isoelectric point of
pH 7·8, optimal
catalytic activity at pH 5–6 and is stable at temperatures
below 50°C. CCN activity increases are observed with
addition of cation cofactors, favouring
Mn2+>Co2+>Ca2+>
Na+>Zn2+. CCN is inhibited by
aurintricarboxylic acid, a nuclease inhibitor. The point
of CCN cleavage on the DNA molecule is between the
5′-phosphate and the 3′-hydroxyl. CCN also
possesses RNase activity.
Although the C. coccodes culture filtrate can
induce the expression of disease resistance response genes
and phytoalexin production in
pea tisue, this inductive activity does not co-purify with
the CCN DNase activity through the Sephacel G-75 purification
step, and thus CCN applied singly does not serve as a signal
for initiating the host defence reponses. Because CCN degrades
both DNA and RNA, it is more probably a nutrient scavenging enzyme.