Ribulose-1,5-bisphosphate Carboxylase/Oxygenase and Polyphenol Oxidase in the Tobacco Mutant Su/su and Three Green Revertant Plants.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) was crystallized from a heterozygous tobacco (Nicotiana tabacum L.) aurea mutant (Su/su), its wild-type sibling (su/su), and green revertant plants regenerated from green spots found on leaves of haploid Su plants. No differences were found in the specific activity or kinetic parameters of this enzyme, when comparing Su/su and su/su plants of the same age, which had been grown under identical conditions. The enzyme crystallized from revertant plants was also identical to the enzyme from wild-type plants with the exception of one clone, designated R2. R2 has a chromosome number approximately double that of the wild-type (87.0 +/- 11.1 versus 48). The enzyme from R2 had a lower V(max) for CO(2), although the K(m) values were identical to those for the enzyme from the wild-type plant. The enzyme from all mutant plants had identical isoelectric points, identical molecular weight as demonstrated by migration on native and sodium dodecyl sulfate (SDS)-polyacrylamide gels, and the same ratio of large to small subunits as the enzyme from the wild-type. The large subunit of the enzyme from tobacco leaves exhibited a different electrophoretic pattern than did the large subunit from spinach; there were two to three bands on SDS-polyacrylamide gels for the tobacco enzyme whereas the enzyme from spinach had only one species of large subunit.Total polyphenol oxidase activity was the same in leaves from the heterozygous mutant (Su/su) and wild-type (su/su) plants when correlated with developmental age as represented by morphology rather than by the chronological age of the plants. There was a marked increase in the soluble activity of this enzyme with increasing age of both plant types and also as a result of varying environmental conditions. Ribulose-1,5-bisphosphate carboxylase/oxygenase activity correlated inversely with increases in the soluble activity of polyphenol oxidase in crude homogenates from which the carboxylase/oxygenase was crystallized over a generation of Su/su and su/su plants. Criteria are outlined for determining if differences in activity of ribulose-1,5-bisphosphate carboxylase/oxygenase are caused by an effect of polyphenol oxidase activity and/or by some other extrinsic parameter.