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Inhibition of Ribulose 1,5-Bisphosphate Carboxylase by a Toxin Isolated from Pseudomonas tabaci. Leola M. Crosthwaite, Department of Plant Pathology, University of Kentucky, Lexington, KY 40506; S. J. Sheen, Department of Plant Pathology, University of Kentucky, Lexington, KY 40506. Phytopathology 69:376-379. Accepted for publication 17 October 1978. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-376.

Toxin purified from culture filtrates of Pseudomonas tabaci by column chromatography inhibited ribulose 1,5-bisphosphate carboxylase activity of Fraction I proteins obtained from tobacco cultivars. Inhibition was reduced when toxin was inactivated by heating at 110 C for 20 min. An inverse relationship existed between toxin concentration and inhibition of enzyme activity. When a mixture of toxin and Fraction I protein was fractionated by gel permeation, Fraction I protein showed a reduction of carboxylase activity, suggesting that the protein may be modified by binding with the toxin. On the other hand, the toxin did not inhibit peroxidase and polyphenoloxidase activities from the same tobacco cultivars. A similarly prepared fraction from P. pisi did not inhibit the activity of carboxylase and oxidases. The results indicate that P. tabaci toxin can inhibit photosynthetic CO2 fixation, and it has a degree of specificity to cellular enzymes; this specificity is not, however, related to resistance or susceptibility of tobacco cultivars to this pathogen.

Additional keywords: Fraction I protein, tabtoxin.