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We have isolated an auxin-regulated cDNA, parB, from the early stage of cultured tobacco mesophyll protoplasts. The expression of parB was observed during transition from G0 to the S phase of tobacco mesophyll protoplasts cultured in vitro. The predicted amino acid sequence of parB cDNA has 213 amino acid residues with a relative molecular weight of 23,965. Nucleotide sequence analysis revealed that parB cDNA has homology to glutathione S-transferase (GST; RX:glutathione R-transferase, EC 2.5.1.18) from several sources including plant and animal cells. When we introduced expression vector pKK233-2, which retains parB cDNA, into Escherichia coli, we could detect GST activity in the parB gene product. Accordingly a significant increase of GST activity was detected in the tobacco mesophyll protoplasts cultured in the presence of 2,4-dichlorophenoxyacetic acid. This is an example in which the function of auxin-regulated gene product is shown to be ascribed to a specific enzymatic activity. As GST, and its substrate glutathione, are shown to be related to cell proliferation as well as detoxification of xenobiotics in plant and animal cells, the role of parB is discussed in relation to the induction of proliferative activity in differentiated and nondividing mesophyll protoplasts of tobacco.
Takahashi et al. (Wed,) studied this question.