The activity of polygalacturonase (PC) has been detected in ripe Mclntosh apples (Malus domestica Borkh. cv Mclntosh) both by enzyme activity measurement and immunoblotting using an antitomato-PC antibody preparation. PC activity increased during fruit ripening and remained steady, or decreased slightly, after 5 months of controlled atmospheric storage. l h e enzyme had a relative molecular weight of 45,000 as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and 56,000 to 61,000 when determined by gel filtration. Viscosity and reducing end group measurements with a commercial pectin preparation showed that the enzyme is endo acting. In RNA and DNA blot hybridization experiments, a full-length tomato PC cDNA hybridized with the apple RNA and DNA, showing the identity of genes encoding the activity of the enzyme in tomato and apple.Structural changes in the cell walls of climacteric fruits that lead to softening of the tissues are initiated by the action of ethylene, the fruit-ripening hormone (Abeles, 1985). The softening of the fruit tissues is the consequence of cell walldegrading enzymes , of which endo-PG (EC 3.2.1.15) was reported to cause the most prominent changes (Crookes and Grierson, 1983). Endo-PG was implicated in tissue softening because its appearance during fruit ripening corresponds to the increase in fruit softening. In a number of cultivars, a correlation between PG activity and fruit softening has been observed (Crookes and Grierson, 1983). Also, application of isolated endo-PG preparations to fruit tissue discs resulted in ultrastructural changes that were similar to those occurring during the normal ripening process (Ben-Arie et al., 1979). Ripening mutants with delayed or decreased softening that were deficient in PG activity Koch and Nevins, 1990) gave further indications that the enzyme plays a major role in the softening of fruits.Results of recent experiments with transgenic tomato fruits, however, are not in agreement with the above assessment. Fruit softening was not affected significantly in tomatoes with down-regulated PG activity (Giovannoni et al., 1989; ' This work was supported in part by a grant from the Cornell showed that when the expression of the PG gene has been up-regulated using the promoter region of another ripeningassociated gene (Schuch et al., 1989), PG production and polyuronide degradation are increased without any detectable effect on fruit softening. These data suggest that although endo-PG activity increases during the ripening process of tomatoes, and the increased activity of this enzyme results in structural changes of the middle lamella, the softening of fruits is a more complex process than anticipated. The ripening process of fruits has been best investigated in tomatoes; however, other fruits such as peaches (Lee et al., 1990), papayas (Lazan et al., 1989), and pears (Pressey and Avants, 1976) also showed elevated activity of an endo-PG during ripening. From biological considerations it is expected that the basis of the biochemi...