ZINC TRANSFER FROM TRANSCRIPTION FACTOR-IIIA FINGERS TO THIONEIN CLUSTERS

The rapid induction of thionein (apometallothionein) by many endogenous stimuli such as steroid hormones. cytokines, and second messengers suggests that this cysteine-rich, metal binding protein participates in an as yet undefined role in cellular regulatory processes. This study demonstrates with DNA and RNA binding assays and in vitro transcription measurements that thionein suppresses the binding of the Xenopus laevis zinc finger transcription factor IIIA (TFIIIA) to 5S RNA and to the 5S RNA gene and abrogates the capacity of TFIIIA to initiate the RNA polymerase III-catalyzed synthesis of 5S RNA. The effect is reversed by the addition of zinc and is not observed in the TFIIIA-independent transcription of a tRNA gene by the same RNA polymerase. In view of the strong tendency of thionein to complex posttransition metals such as zinc. one effect of its enhanced synthesis in vivo could be to reduce the intracellular disposability of zinc and thus modulate the actions of zinc-dependent enzymes and proteins, most notably those of the zinc finger transcription factors.