Wang X., Wurtele E. S., Nikolau B. J.
Department of Biochemistry and Biophysics (X.W., B.J.N.) and Department of Botany (E.S.W.), Iowa State University, Ames, Iowa 50011.
Plant Physiol. 1995 Jul;108(3):1133-1139. doi: 10.1104/pp.108.3.1133.
Regulation of the expression of the gene(s) coding for the 78-kD, biotin-containing subunit of [beta]-methylcrotonyl-coenzyme A carboxylase (MCCase) was investigated in different organs of tomato (Lycopersicon esculantus) plants. The specific activity of MCCase is highest in extracts from roots, followed in descending order by ripe and ripening fruits, 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 leaves. Instead, the difference in MCCase activity between these two organs is directly correlated to the biotinylation status of the enzyme's biotin-containing subunit. Thus, the lower activity of MCCase in leaves is attributed to the reduced biotinylation of the biotin-containing subunit of the enzyme. Consistent with this model, a pool of nonbiotinylated enzyme is present in leaves, whereas the nonbiotinylated enzyme is undetectable in roots. The underbiotinylation of MCCase in leaves is not due to a lack of biotin in this organ, since the biotin concentration is 4- to 5-fold higher in leaves than in roots. These observations indicate that the posttranslational biotinylation of the biotin-containing sub-unit of MCCase is an important mechanism for regulating the organ-specific expression of MCCase activity.
在番茄（Lycopersicon esculantus）植株的不同器官中，对编码β-甲基巴豆酰辅酶A羧化酶（MCCase）含生物素的78-kD亚基的基因表达调控进行了研究。MCCase的比活性在根提取物中最高，其次按降序排列为成熟和正在成熟的果实、茎和叶。根中的比活性比叶中的高10倍。然而，MCCase的78-kD亚基及其mRNA的稳态水平在根和叶中大致相等。相反，这两个器官之间MCCase活性的差异与该酶含生物素亚基的生物素化状态直接相关。因此，叶中MCCase活性较低归因于该酶含生物素亚基的生物素化减少。与该模型一致，叶中存在未生物素化的酶池，而根中未检测到未生物素化的酶。叶中MCCase生物素化不足并非由于该器官中缺乏生物素，因为叶中的生物素浓度比根中的高4至5倍。这些观察结果表明，MCCase含生物素亚基的翻译后生物素化是调节MCCase活性器官特异性表达的重要机制。
