Abstract
DNase activities in Nicotiana tabacum were studied because of their importance and possible effects in molecular genetics studies with tobacco. Leaf extracts were incubated with tobacco, E. coli or pBR322 DNA. Plant or bacterial 3H-DNA incubated with the extracts were not appreciably degraded into acid-soluble products. However, extensive cleavage of all the tested substrates was observed in gel-electrophoretic analyses. The leaf DNase(s) displayed a pH optimum of 5.5, and did not appear to require the addition of any specific ions for activity. Moreover, the activity was inhibited by divalent cations in the order Mg 2+ > Ca 2+ > Mn 2+ > Zn 2+. The enzyme preparation was inactive in the presence of spermin, but not of spermidine. Analysis of the 5′ termini of the cleavage products released by the enzymatic activity indicated that cute occurred most frequently next to A, to a lesser extent next to T and G, and were barely detectable next to C. Thus, tobacco leaf DNase(s) appear(s) to have a distinct base preference for cleavage.
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