Nicotiana tabacum contains a putative mineralocorticoid receptor

Total protein from N. tabacum cell cytosol, partially purified by ammonium sulphate precipitation, contains a 52 kDa protein that tests positive for mineralocorticoid receptor (MCR)-like activity by Western blots, immuneprecipitation and photochemical cross-linking. Binding of RU 26752, a ligand specific to the MCR, suggests 100 fmol steroid bound per mg of tobacco protein; this is equivalent to about 6000 sites per cell assuming a 1/1 stoichiometry. Northern blots of total RNA from isolated tobacco cells, hybridized with a probe specific to mammalian MCR, revealed 2.8 kb and 2.2 kb transcripts, whereas the poly A mRNA was resolved as a prominent message of 1500 bp, preceded by a band of 2200 bp. This appears to be the first ever demonstration where a vascular plant is endowed with a pygmy protein similar to the much larger steroid hormone receptors in the animal world.