Effects of Thionein on Zinc Finger-Dependent Gene Expression

Effects of Thionein on Zinc Finger-Dependent Gene Expression 

32 pmol thionein/oocyte nearly completely abolished the TF/ZM-dependent 5S RNA production while the TF/ZM-independent formation of tRNA and of endogenous messenger RNA remained unaffected. The experiment thus established that thionein introduced into the cytosol is capable of modulating the action of TFIIIA in the nucleus. The lack of any effect of the holoprotein Zny-thionein reemphasizes that it is the metal-chelating power of thionein which is responsible for the inhibition. The fact that a nearly 20-fold excess of thionein over the estimated amount of TFIIIA in mature oocytes (2 pmol) was needed for the in vivo inhibition was accounted for by the considerable stock of extra zinc accessible to the injected chelator in mature oocytes. 

The absence of any noticeable inhibitory effect of thionein on the FFZZZA-independent transcription at the plasmid-encoded tRNA gene by RNA polymerase III and on the overall production of endogenous Xenopus oocyte mRNA by RNA-polymerase II (Fig. 4) indicates that, at the intracellular concentrations attained in these studies, thionein affected the transcription process exclusively at the level of the regulatory zinc finger protein and did not interfere with the transcriptional machineries. As both RNA polymerase II and III are also zinc metalloproteins and inhibitable by high concentrations of the chelator 1,10-phenantroline (cited in Coleman 

input fragment c, major complex. B, Restoration of Spl binding to DNA probe in the presence of thionein by addition of zinc. Lane 1, no extract lane 2, with extract lane 3, addition of 11.7pM thionein lanes 4, 5, addition of 11.1 pM thionein, and then 54, 100pM zinc, respectively. (From Zeng et al. 1991a) 

their refractoriness to microinjected thionein probably reflects their relatively stronger binding of zinc and may thus illustrate the thermodynamic limits of the zinc-sequestering power of thionein (Fig. 1). 

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