- Abstract A nuclease activity was detected in cultured cells of various plants. The enzyme is excreted into the growth medium and degrades single-stranded forms of nucleic acids. This activity has been characterized as an S1/mung-bean-type nuclease, also by its ability to cleave supercoiled DNA only at specific sites. The growth media conditions (low pH and metal ions) favor the activity of this nuclease, while ethylenediamine tetraacetic acid (EDTA) inhibits its activity and polyethylene glycol (PEG) slows the reaction. This activity should be taken into account when transforming plant cells since RNA degrades almost immediately and plasmids are linearized when incubated with the cells. This is especially important since the effect of the topology of certain DNA regions on gene expression is well documented, and linear plasmids were found superior to supercoils for plant cell transformation.