2 ZINC-DEPENDENT STEPS DURING G1 TO S-PHASE TRANSITION

The influence of zinc (Zn) availability on thymidine kinase mRNA conce ntration has been investigated in cells in which production of the mRN A was regulated by either truncated thymidine kinase promoters or by t he SV40 early promoter. Thymidine kinase mRNA concentrations were decr eased by low Zn availability even when the promoter was truncated to 8 0 bp but not when it was replaced by the SV40 promoter. However, thymi dine incorporation by the SV40 cells was still sensitive to lack of Zn , suggesting a second Zn-sensitive process involved in commitment to 5 phase. The increase in histone H3 mRNA production prior to S phase wa s not inhibited by lack of Zn leading to a preferential increase in th is mRNA in exponentially growing cells deprived of Zn.