Termination-Reinitiation Occurs in the Translation of Mammalian Cell mRNAs

Many examples of internal translation initiation in eucaryotes have accumulated in recent years. In many cases terminators of upstream reading frames precede the internal initiation site, suggesting that translational reinitiation may be a mechanism for initiation at internal AUGs. To test this idea, a series of recombinants was constructed in the mammalian expression vector pSV2. Each contained a dicistronic transcription unit comprising the coding sequence for mouse dihydrofolate reductase (DHFR) followed by the gene for xanthine-guanine phosphoribosyl transferase (XGPRT) from Escherichia coli. Various versions of this pSV2dhfr-gpt recombinant plasmid altered the location at which the DHFR reading frame was terminated relative to the XGPRT initiation codon and demonstrated that this is a critical factor for the expression of XGPRT activity in transfected Cos-1 cells. Thus, when the DHFR frame terminated upstream or a very short distance downstream of the XGPRT initiator AUG, substantial levels of XGPRT activity were observed. When the DHFR frame terminated 50 nucleotides beyond the XGPRT initiator, activity was reduced about twofold. However, when the DHFR and XGPRT sequences were fused in-frame so that ribosomes which initiated at the DHFR AUG did not terminate until they encountered the XGPRT terminator, production of XGPRT activity was abolished. This dependence of internal translation initiation on the position of terminators of the upstream reading frame is consistent with the hypothesis that mammalian ribosomes are capable of translational reinitiation.