Polyamines are essential organic polycations with multiple cellular functions relevant for cell division, cancer and ageing(1-3). Regulation of polyamine synthesis is mainly achieved by controlling the activity of ornithine decarboxylase (ODC) through an unusual mechanism involving ODC antizyme(1,4), the binding of which disrupts homodimeric ODC and targets it for ubiquitin-independent degradation by the 26S proteasome(5). Whereas mammals express several antizyme genes(6), we have identified a single orthologue, termed OAZ1, in Saccharomyces cerevisiae(7). Similar to its mammalian counterparts, OAZ1 synthesis is induced with rising intracellular polyamine concentrations, which also inhibit ubiquitin-dependent degradation of the OAZ1 protein(7). Together, these mechanisms contribute to a homeostatic feedback regulation of polyamines(1,7,8). Antizyme synthesis involves a conserved +1 ribosomal frameshifting (RFS) event at an internal STOP codon during decoding of its messenger RNA(6-10). Here we used S. cerevisiae OAZ1 to dissect the enigmatic mechanism underlying polyamine regulation of RFS. In contrast with previous assumptions, we report here that the nascent antizyme polypeptide is the relevant polyamine sensor that operates in cis to negatively regulate upstream RFS on the polysomes, where its own mRNA is being translated. At low polyamine levels, the emerging antizyme polypeptide inhibits completion of its synthesis causing a ribosome pile-up on antizyme mRNA, whereas polyamine binding to nascent antizyme promotes completion of its synthesis. Thus, our study reveals a novel autoregulatory mechanism, in which binding of a small metabolite to a nascent sensor protein stimulates the latter's synthesis co-translationally.