The localization of IGF - I and IGF - I 1 mRNA in the developing embryo by hybridization histochemistry

Zinc is essential for growth and reproduction of animals and i t has been suggested that the most critical requirement for Zn is for alteration in the genetic expression of cells (Chesters, 1978). However, in EDTA-treated lymphocyte cultures, lack of Zn delayed maturation of 32s ribosomal RNA precursor to 28s ribosomal RNA and reduced survival of the 28s RNA (Chesters, 1975). This alteration in the post-transcriptional processing of ribosomal RNA was difficult to reconcile with a primary effect of Zn on gene expression. Recently, however, the protein TFIllA which is specifically required for transcription of the 5s ribosomal RNA gene has been shown to contain up to 1 1 repeats of a Zn-binding domain involved in attachment of the protein to the gene (Hanas et al. , 1983; Miller et al. , 1985). While most of the RNA components of mature ribosomes are derived from the 45s precursor, 5s RNA is synthesized independently and then incorporated into the large ribosomal subunit. If lack of Zn limited TFIIIA sufficiently to impair expression of the 5s RNA gene, 28s RNA no longer able to be incorporated into large ribosomal subunits might be more vulnerable to degradation. The present experiments were designed to determine whether the Zn requirement for TFIIIA function does restrict the synthesis of 5s ribosomal RNA in a Zn-deficient rat. Both tRNA and 5s RNA are synthesized by RNA polymerase I l l presumably from the same precursor pool. If this