PROTEIN PROCESSING AND AUXIN RESPONSE IN TRANSGENIC TOBACCO HARBORINGA PUTATIVE CDNA OF ZEATIN O-XYLOSYLTRANSFERASE FROM PHASEOLUS-VULGARIS

Zeatin is rapidly metabolized to O-xylosylzeatin in Phaseolus vulgaris seeds. The zeatin O-xylosyltransferase mediating this conversion, a 5 0 kDa protein, occurs mainly in the endosperm, both in the cytoplasm a nd the nuclei. A monoclonal antibody specific to the enzyme was used t o isolate cDNAs from an expression library derived from P. vulgaris se eds. Two highly homologous, full-length cDNAs were isolated. The ORFs encode proteins of 69 and 67 kDa, respectively, with 90% homology at t he amino acid level. cDNA-encoded protein obtained from in vitro trans cription/translation was processed to protein of 50 kDa by bean endosp erm extract. Transgenic tobacco plants harboring the larger ORF under the control of the CaMV35S promoter were more sensitive to the auxin N AA than control plants. The symptoms included leaf chlorosis, restrict ion of root elongation, and eventual cessation of growth. The antigeni c preprotein was processed, and labeled zeatin was converted to O-xylo sylzeatin in transgenic plants grown on NAA-containing medium. Analyse s of independently transformed families indicated that the presence of the transgene coincided with the increased auxin sensitivity and prot ein processing correlated with the manifestation of auxin-induced dama ge. These results suggest that posttranslational processing regulates enzyme activity, and offer the possibility that cytokinin-auxin balanc e may be affected by stimulation of cytokinin metabolic enzyme activit y by auxin.