REGULATION OF BETA-METHYLCROTONYL-COENZYME-A CARBOXYLASE ACTIVITY BY BIOTINYLATION OF THE APOENZYME

Regulation of the expression of the gene(s) coding for the 78-kD, biot in-containing subunit of beta-methylcrotonyl-coenzyme A carboxylase (M CCase) was investigated in different organs of tomato (Lycopersicon es culantus) plants. The specific activity of MCCase is highest in extrac ts from roots, followed in descending order by ripe and ripening fruit s, stems, and leaves. The specific activity is 10-fold higher in roots than in leaves. However, the steady-state levels of the 78-kD subunit of MCCase and its mRNA are approximately equal in both roots and leav es. Instead, the difference in MCCase activity between these two organ s is directly correlated to the biotinylation status of the enzyme's b iotin-containing subunit. Thus, the lower activity of MCCase in leaves is attributed to the reduced biotinylation of the biotin-containing s ubunit of the enzyme. Consistent with this model, a pool of nonbiotiny lated enzyme is present in leaves, whereas the nonbiotinylated enzyme is undetectable in roots. The underbiotinylation of MCCase in leaves i s not due to a lack of biotin in this organ, since the biotin concentr ation is 4- to 5-fold higher in leaves than in roots. These observatio ns indicate that the posttranslational biotinylation of the biotin-con taining subunit of MCCase is an important mechanism for regulating the organ-specific expression of MCCase activity.