Inhibition of Ribulose 1,5-Bisphosphate Carboxylase by a Toxin Isolated fromPseudomonas tabaci

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Abstract
Toxin purified from culture filtrates of P. tabaci [P. syringae] by column chromatography inhibited ribulose 1,5-biphosphate carboxylase activity of Fraction I proteins obtained from tobacco cultivars [Nicotiana tabacum ''Barley 21'' and ''NC95'']. Inhibition was reduced when toxin was inactivated by heating at 110.degree. 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. The toxin did not inhibit peroxidase and polyphenoloxidase activities from the same tobacco cultivars. A similarly prepared fraction from P. pisi [P. syringae] did not inhibit the activity of carboxylase and oxidases. P. tabaci toxin can inhibit photosynthetic CO2 fixation, and it has a degree of specificity to cellular enzymes; this speificity is not related to resistance or susceptibility of tobacco cultivars to this pathogen.

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