Expression of the auxin-regulated parA gene in transgenic tobacco and nuclear localization of its gene products.

An auxin-regulated gene, parA, comprises a gene family consisting of a handful genes which respond to various signals. Although Droog et al. (Plant Mol. Biol, 1993, 21, 965-972) postulated that the parA-related genes belong to the family of a cytoplasmic enzyme, glutathione S-transferase (GST), we detected a low level of GST activity in the parA products, whose value was below 1/30 of that of parB products encoding tobacco (Nicotiana tabacum L.) GST. Immunofluorescence studies using an antibody against parA protein revealed that the subcellular location of parA protein is the nucleus in cultured tobacco mesophyll protoplasts, while conventional GSTs' including the parB product were primarily located in the cytoplasm. Confocal laser scanning microscopy of tobacco BY-2 cells showed that the parA product was confined to the nucleus, but was excluded from the nucleolus. In addition, exon/intron organization of the parA family was appreciably different from that of conventional GSTs including parB. Furthermore, the parA protein is much more similar to a 24-kDa protein of Escherichia coli that is reported to bind to RNA polymerase. These different characteristics of parA compared with to the conventional GSTs, indicate that parA protein would have distinct functions, such as involvement in transcription, rather than functioning as a conventional GST. Transgenic tobacco plants that carried the parA promoter fused to a beta-glucuronidase gene were used to show that the parA gene is tissue-specific and also under developmental control.