THE HALF-LIFE OF C-MYC MESSENGER-RNA IN GROWING AND SERUM-STIMULATED CELLS - INFLUENCE OF THE CODING AND 3' UNTRANSLATED REGIONS AND ROLE OF RIBOSOME TRANSLOCATION

c-myc mRNA contains at least two discrete sequence elements that accou nt for its short half-life, one in the 3' untranslated region and the other in the carboxy-terminal coding region (coding-region determinant ). To investigate the function of each determinant, one or both were f used in frame to portions of a gene encoding long-lived beta-globin mR NA. Each chimeric gene was stably transfected into HeLa and NIH 3T3 ce lls and was transcribed from a constitutive cytomegalovirus promoter o r from a serum-regulated c-fos promoter, respectively. The steady-stat e levels of the chimeric mRNAs in exponentially growing HeLa cells wer e compared, and their half-lives were measured by two independent meth ods: (i) in actinomycin D-treated HeLa cells and (ii) after serum addi tion to starved 3T3 cells. By each method, mRNAs containing either ins tability determinant were less stable than beta-globin mRNA. mRNA cont aining only the c-myc 3' untranslated region was not significantly mor e stable than mRNA with both determinants. In a cell-free mRNA decay s ystem containing polysomes from transfected HeLa cells, mRNA containin g the coding-region determinant was destabilized by addition of a spec ific RNA competitor, whereas mRNA containing only the 3' untranslated region was unaffected. When a stop codon was inserted upstream of the coding-region determinant, the chimeric mRNA was stabilized approximat ely twofold. These and other data suggest that degradation involving t he coding-region determinant occurs most efficiently when ribosomes ar e translating the determinant.