A soluble auxin-binding protein from cultured tobacco tissues stimulates RNA synthesis in vitro

When the soluble auxin receptor from tobacco callus was isolated according to H. Oostrom et al. (1975, FEBS Lett. 59, 194-197; 1980, Planta 149, 44-47) a high polyphenol contamination in the receptor preparation was observed. We developed a new isolation procedure, which drastically reduced this contamination. The receptor, which was partially purified on Sephadex G-200, exhibited the same time- and temperature-dependent binding kinetics as described before (Oostrom et al. 1975, 1980). The Ka for indole-3-acetic acid (IAA) at 25°C was about 1.6·10(8) M(-1) and the number of binding sites varied from 0 to 2·10(-13) M mg(-1) protein. Addition of partially purified receptor preparations to isolated tobaccocallus nuclei resulted in an IAA-dependent stimulation of transcription, which was not observed with similar preparations that did not contain detectable amounts of specific IAA-binding sites. The average stimulation in the presence of 1 μM IAA was 42%; it was achieved by an increase in RNA-polymerase-II activity. The stimulation was not dependent upon the presence of 1 μM IAA during the isolation of the nuclei.